1.Are liquid(bought) yeasts any better?
2.Is harvesting yeast from a brew just a money saving exercise?How do you know if the yeast cells are all live?
3.Can harvested yeasts be used on any brew and not just the same type?
4.Are these dumb questions?
1. Maybe, some people find that bitters made with a liquid strain will be a lot more flavoursome than those with a dry and like wise similar opinions with belgian strains, the liquids in general seem to be more characterful if that's what you're after, but it's all anecdote and opinion.
2. Kinda, but is also mean you can learn how a strain behaves and if you re-use it enough it could become a house strain which may react differently than the original for your set-up. I go for the over-building starters method, so I grown up an extra 100 billions cells, decant that portion of the starter into a kilner jar and store in the fridge. When I next want that yeast I (over)build it up in another starter.
3. As above, some styles need their own yeast so it depends what you brew. I currently have the following in the fridge, English, Lager, Saison, Abbey, Kveik. I may only brew 1 more batch from the abbey and I'm not sure if I want to keep the lager strain about. I'm enjoying the english one (Wyeast 1469 West Yorkshire) it's done a bitter, a porter and a stout thus far and I have a few more planned for it.
4. Nope, liquid yeast costs a fair bit more than dry and is extra work to keep going if you don't want to buy fresh every time so make sense to ask if its worth it.
Good luck.