That's how I looked at it in the end (when I got over the shock of the quotes). We had a full-service installation, and it was well worth paying a bit more to know that there was a project manager ensuring that the various tradesmen turned up when they were supposed to (and presumably he could have got another one if there had been a delay), and that everything went according to plan. One person being a few days late can throw off the whole project if person B has to wait for person A to finish before he can start, and they always have other jobs lined up, so can't hang around forever.
We had a couple of small glitches which were sorted out straight away, such as the bath not fitting because our walls aren't straight. It was about a centimetre out on one side, but the bath had to be changed - if we'd bought it ourselves we'd be stuck with it, or would have had a lot of hassle sorting out a replacement, by which time the tradesmen might have had to move to another job. We went away when it was going on, and the PM rang me at close of business every day to let me know the progress, and ask if I had any preferences where options arose. He designed the bathroom much better than I would have done, and considered things like lights in the niche by the bath, which I love, as the overhead lighting is a bit harsh, and sitting in the bath with just a couple of lights in the niche is so relaxing.
We've done things like this ourselves in the past when we were younger and had to count every penny, but it was so much easier and so much better getting it all done by professionals. And yes to insurance. Floods cost a fortune, and if they are a result of a DIY job you will have to sort it out and hope your house insurance covers it. Knowing the company has professional indemnity is reassuring.
Obviously, as with everything, much depends on budget and your own expertise, and you can absolutely save a lot by doing it yourself, but if budget allows, I would recommend a proper bathroom company over a jobbing plumber and a B&Q bathroom suite or a DIY job. Oh, and get a company with a showroom over a one-man band who can just block your number and disappear if you have complaints. A showroom lets you look at items before deciding, and also (more importantly) means that they are committed to being there and dealing with reviews and so on. Again, overheads add to the cost, but could save money in the long run if things don't go according to plan.