I can tell you that, for myself, having one dev has
definitely resulted in
much higher Quality and a usable deliverable.
Most of my OSS projects are 1-file UI widgets (but with a lot more testing code than implementation code). I do, however, have a couple of monsters.
Because of my experience (which includes many mistakes), I am able to develop projects of a far more ambitious scope than the run-of-the-mill developer. The one that I'm working on now (not OSS), for example, is one that would usually be done by a heterogenous team.