Incidentally, there has been some discussion of that same topic before.
A -2 wouldn't stop them from also casting a spell giving a recovery roll bonus, unless you specifically rule them incompatible.
If they want fast healing, they should pay for it. In vis or otherwise. Otherwise you'd think it would be the preferred healing method in the Order already.
Plus if you can make the body recover from wounds more quickly, it should be possible to make it recover from fatigue more quickly, too, no? There goes the Limit of Energy.
Hmm... make the accelerated recovey painful (a season's worth of pain in one week!) and thus have psychological side effects (lower Brave and Loyal scores)? Have it give one Aging Point, without a roll? Make a 6+ Stamina check or gain one Decrepitude? 
Warping is always fun, too. You can make the target more susceptible to bad air, humor imbalances, disease spirits and other demons: while the body is busy fighting the designated wound, its defenses are down on the other fronts.
And the recovery of the other wounds is suspended? Or you do have to check whether they worsen (cannot improve).
Guaranteed vis-less fast recovery means that if a mage ever gets wounded, he can recover without his season's work being affected, and not have to pay for it. Non-ritual fast recovery means (as Tuura pointed out) that you can the effect in a magic item. It means that if your shield grog get slapped down in the middle of a story, you can bring him back up without delay. It means you don't have to reach for the higher-level instant-healing spells either (avoids warping, don't need overly high CrCo totals).
Heck, just ask him.