You make a good point that you have to look at the profitability, not just the repeat user. A repeat customer who cost you a fortune to acquire might or might not be profitable. A repeat customer who uses the system to max capacity might or might not be profitable. (For example, the Netflix user who gets her DVDs in the mail the day after she gets them.) Someone who’s on the phone with support every other day may or may not be profitable.

I’d add into that, though, referral behavior. Someone who’s a high-maintenance PITA but who sends dozens of customers your way can be worth the effort–as long as she’s not burning out your staff.