[kweingar]

With GPL you don't have to actively work to upstream your patches, but in practice you can't withhold your patches from upstream. If you add a feature, they get to have it too.

Unlike permissively licensed software, where you can add proprietary features.

[pabs3]

Depends how savvy your users are, and what your users lose if they do send your patches upstream. For example, GRSec or RedHat both drop you as a customer (so no security updates) if you republish their patches publicly. Or a paid iPhone app's users probably wouldn't know what source code is, let alone where/how/bother to republish it for the benefit of other users.