It's also something that can limit the adoption of your product. I've worked with companies in the past have either refused to use allow GPL software, or would have a significant audit process you'd have to go through to be able to use it.
http://zeromq.org/area:licensing
Additionally, patent license provisions for using the shared code seem almost mandatory given the patent trolling we're seeing of both suppliers and users of whatever they manage to get through a patent clerk. All the FOSS licenses should try to include some protection there against beneficiaries suing derivatives.
Only with AGPL, and only if the competitors expose APGL-licensed software for interaction directly.
Suppose they've started offering Sentry-with-a-twist, how can you know they modified Sentry code? They may as well have written a separate software running in a different stack, but interfacing with Sentry and with the users and providing the twist.