"Do we really need all the little incompatible changes, new UI shells, multiple marketplace stores and multiple messaging clients and crapware on Android OEM phones?"
Yes, we do. If different people did not try doing new things (or just old things done differently), there wouldn't be as much progress, and we would all be poorer for it. Sure, some things stick, some don't, but that's how it's supposed to be.
The problem is the motivation of most of these differences is not improvement but just differentiation. They are literally only different for the sake of being different, for marketing purposes.
Yes, we do. If different people did not try doing new things (or just old things done differently), there wouldn't be as much progress, and we would all be poorer for it. Sure, some things stick, some don't, but that's how it's supposed to be.