Think of it as a scrapyard; within you'll find hundreds of vehicles that don't work but inside them are some goodies that one could strip down and throw into your existing vehicle. The main thing is that you'd have to be prepared to wade through some garbage to find what you need...
Take this concept to a game engine level; if someone were able to hunt through a trashed engine and pick out a nice resource manager, it'd save them the time in writing a full version for their code; or they could chop it up and work with it as they pleased, or at very least it'd give them a few ideas for implementing their own (even if it's what NOT to do).
The main issue I see is that many engines have different licenses, including the disgustingly viral GPL license - if you were to use a small component from one of these, the rest of your project - including the stuff you've salvaged from other projects - will be forced to be GPL'd. Yuck.
Still, it opens up my mind to thought a little. I have quite a few rotting corpses stashed around my harddrive and various CDROMs; everything from small failed game engines, to basic failed graphics engines, to small scale testbed ideas, managers and utility classes. Some of them must be useful to others, or even myself - afterall who knows what lies in long forgotten graveyards.
I still don't feel ready to come back to full-scale development again; I've been tinkering compiling GCC for sh-elf but that's about as far as I've gone. It's pretty weird - I want to code but just can't; it's as if the very ability has been sucked out of me yet the desire still remains. It'll be back, the long haitus up a mountain is doing me good, I can feel it.