I'm totally baffled with this unit.
Yes it is the only minelayer cruiser in the game, yes it is supposed to be used offesively (like everything Vasari), but seriously, wtf is the use of this?
I tried to experiment a bit with Ruiners but can't figure out any clever uses of them.
The best I could do was cleaning Terran planets around 10th minute or powerleveling my cap.
What I did was tanking the defenders in one end of the gravwell with my cap and setting up two clouds of mines in the other end, then running the defenders through the mines for quick carnage (AI ships won't target the ruiner, flaks have only 30 secs to down the mines - if they're far way, there's no chance; also, neutral AI defenders have no scout frigs).
If you have 2 ruiners, you can completely wipe a Pirate Base's fleet by 15th minute (that's ~3-5 levels for a fresh cap).
Still, I can't see any use of the ruiner in a player vs player match. Mines themselves can be useful, but I can't seem to find any way to exploit (as in use, not abuse) its mobility.