If someone publishes a bug fix, or finds a cleaner way of doing something, I'd like to be able to incorporate it without being accused of having stolen their IP. That's the main thing.
GPLv2 requires modified public distributions to also be under GPLv2, so you could both see the fix/change and use it (in the sense that you won't be infringing on copyright).
GPLv3 adds patent protection, so someone adding a fix/change that they have also patented explicitly grants a license for that patent.
I think GPLv3 is the most comprehensive way to keep fixes/changes of others available for reuse.
http://www.apache.org/licenses/LICENSE-2.0
OTOH, it's a big, long license, and perhaps the BSD license would be simpler, which is always an advantage.