There wouldn't be many of the features LimeWire has if it would depend on open source developers to work on it in their spare time. - With six developers, they are working (I guess) about 300 man/hours on LimeWire every week, - you would need at least 20-30 open-source developers to do the same work.

Don't forget that many successful open-source projects have full-time developers working on them, e.g. OpenOffice (sponsored by Sun), Mozilla (by AOL/TW), the Linux Kernel (paid by various Linux distributions, IBM, HP, Sun...).

The GIMP for example is done by open-source developers in their free time, and all they manage is a point release every other year.