no support from me though
That will be difficult. Although you wouldn't support the apps, those apps can have side-effects that bring the whole OS down (e.g. memory leaks). So you are in the support loop again!
I had a problem with someone using the facebook app for BB. Since he has installed it, the mails older than 30 days haven't been deleted. So the FB app had a side-effect on his business mails on the BB.
So expect a lot more support if you allow third-party apps, even if they are certified by RIM.
Our policy is very strict. We don't allow any third-party apps, except the ones we we distribute ourselves via software policy (e.g. Bloomberg for BB).