Ben,
Yeah, if you wanted to write code to be compatible with non-GHC compilers, yes, you'd have to supply the methods anyways.
This is pretty much what I do with anything remotely portable that I write. I put in default signatures so folks who aren't concerned with portability can get ease of use and then i bother to put the default definitions for my own methods unless i'm writing something highly GHC-specific that can't work with another compiler anyways.
I'd be perfectly okay with just saying +1 to the proposal as it stands. DefaultSignatures have somewhat annoying nuances. e.g. you have to have the subclass present where you define the superclass, which means internally Functor, Applicative and Monad would need to live in the same module somewhere, before being re-exported from their current homes, and then missing definitions upgrade from a simple warning to a full error about not being able to find the instances required for the default signature.
I mentioned it mostly to remind folks that it does provide an alternative to some of the plumbing used in this proposal -- not intending to gum up the works!
-Edward