It's true that writing an engine is a learning experience, I personally develop my engine because I enjoy experimenting with new techniques, I designed it's scope so I could learn about aspects of their design that I was interested in myself. So it's not going to offer everything you need for a game, only a small subset. but enough to decrease the development time of a game. I think that's the best you can hope for unless you've got a big team of people, and I do think it would be better to have one or two really good community contributed engines than all these talented developers, reinventing the wheel independently.

It's difficult to draw devs together in this way because I suppose that many of them also enjoy learning new engine techniques and improving their engines.

But if there's more than two devs that are just trying to create a kick ass engine for people to use, it'd be a shame to duplicate effort.