The 1 or 2 kind of ship fleet... I don't agree.. Lets take Aircraft, you have an Interceptor, you have air superiority, you have bombers, you have escorts, Etc. Etc. Can 1 plane do everything? Sure.. but some it will do quite badly, and going against say like a dedicated air superiority fighter, it would get its ass handed to them..
So in Starfleet ships, can you do 1 or 2 basic designs? Sure! but you'd also get the same thing.. mediocre at most things..
Having that type (or types) of ships for say a patrol fleet, where there realatively small, patrol a border, or make house (planet) calls, you don't need a large explorer type. but you do need something that is fast, and realitively well armed for its size. Would it make for a good explorer ship? It has sensors, but nothing that is specialized, etc.
Can the same design be outfitted differently? Sure, but your main hull would be generic and not specialized, and you could only go so far on changes.
So just my opinion, different size, shapes, etc for different missions