Transgressions resulted in them declaring war on me a few times, I can't exactly call that useless. As Romans I sometimes ask for military access if one of the senate missions is for some far away city that I have to get to fast.

It would be nice to be able to force their troops to leave your area if you send your diplomat to them. Usually perfectly good alliances are broken because the AI troops "felt like it" when they were next to your capital. Ouch.