[Meursy]

Yeah, it's true that being diseased increases your chances for further affliction accumulation, and it always sucks to lose a mage to afflictions, but you generally get quite a lot more usefulness out of your mages before you have to kill them off in battle if you give them regen. It's almost always worth the investment if you can afford the gems.

A possible exception could be old age mages. I've noticed that mages who are already in old age and diseased (e.g. Abysian ones) tend to accumulate those extra afflictions at a significantly higher rate than non old age mages. Whether this is only due to the Late Winter affliction/disease check or applies every turn I don't know. I'm talking in a timescale of a year to two years game time (which is already a lot for your typical MP game). This info might be applicable to longer timescales but I'm not as sure.

I'm still giving old age disease mages at least a Lycanthrope amulet if possible though!

And out of those afflictions that LDiCesare mentioned, only feeblemind, and to a lesser extent mute, really hurt your mages (the only commanders worth worrying about generally). Obviously physical afflictions might prevent the mage contributing in battle, but you'll have plenty of affliction free mages for that. Keep your cripples in the lab!