I agree. Look at nginx vs apache - competition with nginx has done a lot to improve apache. Merb did more to improve Rails than anything else has. New projects with different design centers give you the flexibility to try out approaches that wouldn't be possible in the old project.

The thing that sucks is people taking sides and getting all bent out of shape over things. Even the people behind "competing" projects can cooperate in their work. Ruby MRI, JRuby, Rubinus, Maglev - all these supposedly competing projects created collaboration on RubySpec and other shared pieces of technology. When John Trupiano and I collided over Rack::Rewrite vs Refraction, we just talked on IRC and had a friendly chat about our different approaches and motivations, and how features in one project influenced the other. Nobody got mad or hated anyone over it. Even major political rivals can be close friends - look at James Carville and Mary Matalin (or don't, if you value your peace of mind).