Eh, they're really just meant as a place to save / advance the in-game clock. And they're even less integral now than in past GTA's since you can quick save anywhere on the cell phone.Hmm, that seems weird to me. What's the point of a safe house if you aren't safe? And plus you can't fight back at all either. It's almost like they are the most unsafe places to be.
Not being able to fire back is odd, though. Seems like an oversight R* just forgot about.