Yet I still think that copyright transfer makes sense in some cases. Namely, when your contribution is "trivial" (think a bug fix). What's the point of keeping the copyright of a handful of lines of code? It only serves to complicate matters for everybody (maybe that's what Marc was referring to, otherwise I cannot make sense of the plant analogy).
Substantial contributions are a complete different thing, though. I don't think that copyright assignment is a good idea in such cases, unless you truly trust the receiver.