Now that I have your attention with my silly newspaper headline-style thread subject, I'll concede that mobile repair ships probably aren't poo, it's more likely that I'm not using them properly. However...
I just built my first one and sent it to accompany my strike fleet into an enemy world. The problem is that they move really slowly, speed 8 I think which is the slowest I've seen anything move, and so they couldn't keep up with the fleet. Of course it'd be pointless to have the fleet move at the same speed as the repair ship because then we'd crawl all over the map.
I suppose the alternative is to park it near the middle of the map, or as near as is safe, and send the fleet to it for repairs between battles. But that wouldn't really make it very useful either.
How do you guys use yours?