I have a bit of a fiery toddler, and yes it is sometimes easier to let him do things than just always say no, but there are some things that are non negotiable. So, he would love to play with my make up and tubes etc, but I have taught him that they are "mummy's things" and not toys, so he doesn't. Same with toothpaste, but he's allowed to splash in the sink with his toothbrush even though it makes a mess because it's not causing any long term cost or damage.
He also loves emptying my wallet which is a nuisance, but isn't the end of the world, so I let him as it's easier than wrestling it away from him. If I really cared I would put it out of his sight.
He can play with our kitchen utensils but (after telling him about 500 times) they don't go in his mouth otherwise they get taken away, as frankly I can't be arsed to wash them after each play.
I actually think I am a fairly relaxed parent and try to think "why not" when he wants to do something, but then I have some strong non negotiable rules like no hitting, hold a hand near the road etc.
If you don't have a child who is likely to blow up when you say no, I think it's hard to understand where we're coming from! If "no" is met with screams and sobs it is a lot easier to "pick your battles".