I've never taken a repair module so far and was wondering the same thing. Good to know.
I can't see taking a repair module now if I can squeeze another item on there instead.
I have used them a bit and so far they seem kind of good. Ships take forever to repair, especially late game when hit point pools are very large. If you have these you can attack weak opponents that don't do any damage in between major battles and get a boost to your ship repair. They really need to add some way to repair ships faster. After a ship gets low in HP I find many times they are out of the entire war getting repaired. They should add a large repair boost to shipyards if you park damaged ships there.
EDIT: I just realized you were talking about the mass repair modules, I haven't tried those. I was using the individual repair modules you have to install on each ship.