That wasn't very helpful of the assistant to laugh.
Apologies in advance because this will be a bit of a long one.
Okay so firstly - do you need a car seat that goes on your pram - no, not really. Many people find this a convenient option, but it's definitely not necessary, especially if it doesn't appeal to you. I didn't have this for my first baby and it was really no issue at all to transfer him into the pram, as soon as you start walking, the movement puts them straight back to sleep. Although I had a compatible car seat with DC2 and 3, I have also transferred them at times because they shouldn't be in the car seat for hours and hours anyway, and found exactly the same. Worst case scenario, you sit in the car for 15 minutes or so to let your baby get a bit of a nap if you think they will wake up. Really no big deal at all.
Whether it is better to get a car seat for 0-4 years vs 12-15 months - this is a completely different question and there are a couple of things to consider, as well as being different reasons people usually want to do this, the big three being: Cost, eco-friendliness, and decision fatigue.
But first of all the big thing most people don't necessarily realise: 0-4 years is an average. Those car seats are designed to last right up to the fourth birthday for a child bang on the 50th centile, which means that any child that is taller for their age will outgrow the seat sooner, and any child who is shorter for their age will fit into it for longer. A 75-90th centile child will normally outgrow these seats by around age 3-3.5 and 99th centile, often before their third birthday. (50th centile means that the child is taller than exactly 50% of children their age and gender).
These days, the fourth birthday is around the earliest that most experts would recommend you move your child to a booster seat (aka the next stage of seat). That means that there's around a 50/50 chance that this seat simply isn't going to last you for the full second stage, meaning you'll need to buy another harnessed seat (up to 25kg is the next weight category). This might not matter to you, but it's important for looking at the three reasons people decide to buy a fixed seat from birth. And unfortunately it's not really easy to predict in advance whether you're likely to have a tall child or a short one, unless you know you and your partner are both very tall or very short or you know you're predicted a big baby or you know you're expecting a premature birth. Most people don't fall into those categories.
OK so to go back to the reasons:
Cost
It can seem like you save money by buying one seat covering two stages at once, but if you are unlucky enough to fall into the "tall child" category then you will find you need to buy three seats after all:
0-4y seat > bigger harnessed seat > high backed booster seat
Whereas if you start out with an infant carrier you can wait and see what height/weight centile your child is on and guarantee that you'll only ever need to buy three seats.
Infant carrier > up to 18kg -OR- up to 25kg seat > high backed booster seat
And cost-wise, well, it really depends what you buy - you can easily spend over £400 on infant seat + base combo these days if you get pulled into the hype around the luxury seats, which I honestly think is a lot of money to spend for a year - but you can also still pick up a basic and decent infant carrier for less than £100, especially if you don't feel the need for a base. 0-4 years fixed seats can cost anywhere from £150 to £500 (I'd be wary of the really really cheap ones) so it really depends what features you're looking for and where your budget is, but most of them are around the £200-300 mark.
Eco-friendliness
Basically the same argument applies here - and arguably, more materials go into your 0-4 years' seat than a simple infant carrier without base.
For both the cost and avoiding unnecessary consumption argument, it's worth considering whether you plan to have a second child within that first 4 years and whether you would therefore need a second stage-2 car seat. If you are careful to choose a longer-lasting infant carrier, then you could in theory keep your second baby in there for over a year which might just be enough time for your older child to gain the maturity necessary for a high backed booster seat with seatbelt. Or it might be that you're planning a smaller gap and you'll need to buy two stage-2 seats anyway - in which case - the combination as follows works brilliantly for passing down between children, and can actually completely negate any waste:
0-4y seat > larger harnessed seat OR second harnessed seat OR harness/booster seat > high backed booster seat
Decision fatigue
I understand that choosing items for your baby can seem overwhelming and the idea that you won't have to keep buying car seats every few years and you're just set until they are four (or even twelve) is really enticing, but I'd definitely think about the weight/height issue. Yes you will definitely be putting off the need for another seat for several years either way. But I would go in with the assumption that you'll need to buy another car seat in 2.5/3/4 years' time, whether because of a sibling or because the seat is outgrown.
And the ones that go up to 12 years - don't bother with them. They're a false economy, they almost never get a good fit for a newborn, 12 years is an absolute age in car seat safety, and you're shooting yourself in the foot because for 2/3 of the seat's use (years 4-12) you're stuck with a bulky, annoying, outdated seat that's weathered the toughest of the sticky toddler years. You won't WANT to keep using the same seat for 12 years. You honestly want to replace the giant harnessed toddler seat with a booster as soon as possible (safety permitting of course).
If you do want to get a fixed seat what should you consider?
First, it's absolutely essential to get a fit check in your car before you finalise any purchase. Not all car seats fit all cars, you will definitely need to cover the rear facing stage for quite some time, (actually, the longer you rear face the better) and these seats take up more space than infant carriers so check the front seat space with the seat installed. Check the angle. Most of these seats are isofix and if your isofix points in your car are badly located this can cause a problem and tip the seat right back which is simply not suitable for a newborn. Try the seat without the newborn inserts too - sometimes the angle can be really steep for a 6-12 month old baby - toddlers are generally OK with a more upright angle.
Not all fixed seats are really designed with newborns in mind. See if the shop you're looking at has a newborn or small sized doll to try in the seats, or take a teddy that fits into a newborn sized vest. Test whether the safety harness really looks like it will go small enough to secure a newborn. Not all of them do, as sometimes they are designed with the assumption you'll move over at around 6 months old.
If you are concerned about the 50th centile problem or you are quite sure you are likely to have a taller than average child and think it's unlikely you will want to hand the seat down to another baby, you might want to consider seats that already have the 25kg capacity built in. There are only three that go from birth to 25kg - Axkid One+, Avionaut Sky, BabyAuto Dupla. I don't know how the latter two accommodate newborns so you'd need to research that, but the Axkid One+ is great (it is not cheap, though.)