[tremon]

Make your competition share all of their work while you don't have to.

Can't your competitor then just release their contributions as AGPL? Of course, you're free not to accept their contributions, but under the AGPL you can no longer merge their contributions back into your commercial product without having to share all of your work too.

(Yes, usual caveats about the scope of "the work" apply. But that applies just as much to your competitors as to you)

[mperham]

Under the GPL, contributions have to use the same license: GPL.

[cxr]

Not true. GPLv3 provides one exception, and that is that in conjunction the the provision in AGPLv3, you may combine code licensed under each, and the combination is subject to the terms of AGPLv3.

(Whether this has any significance to context that the other commenter's question was in response to is a different matter.)