I concur with Serban. **
I esp. like:
"...grab the uppity mage by the collar, and introduce her to your more stabby friends."
** Meaning, please don't vote for my answer unless you've already voted for his. :D
Auras will be degraded. By mages who want to do things, or by stabby people for self-protection vs. hellfire. Esp. since it takes so long to regenerate.
Also, I disbelieve the ability to learn something that you can't use very often (once a decade? not happening). Learners aren't optimized and will make Sorcerer's Apprentice mistakes. Endless goto loops, for an equivalency.
Alternative idea: If it merely took days-to-weeks to regenerate, you'd still have some problems - but you'd actually get to see magic more than once a decade. You'd get first-to-shoot effects; unless you had a way to block other mages from using spells (or, to lock up the aura for a bit; "noise in the aether, can't spell right now"). Or block them as they cast/right after they've cast their (usual*) spells - so the mana isn't lost - ie: allowing you to actually have a wizard's duel, which your setup prevents from ever happening (or, happening for more than 1-3 shots). * Some spells might be instant effect, and unable to be blocked.
Back to your question:
Does detecting magic users (using a spell, which is the only way I assume you can accurately do so) seriously degrade the local area? That's the only way a council of mages could get control of an area; ie: hope nobody nerfed the aura, use magic to detect anyone who's magic-capable, and keep ruthless control with stabby friends and monitoring anyone who comes close to the area in order to get a place where you can do anything magical.
How big are areas? And do they overlap/spill into one another? ie: If you hellfire an army does it take out a bigger area - say, miles around (which is what I'm envisioning from your question) than just the one you had an army in (which I'd say is several thousand square feet (depending on how big your army is, of course))?
If areas are small enough to be protected, and powerful enough to do stuff - then you might have some interesting effects: ie: if a mage can defend his tower, and his work-room is an 'area' that has enough power to do stuff, that can't be taken out by someone two blocks away deciding to light his cigar with hellfire... then it might be possible to work a story/campaign with this and have mages be something other than meat-sheaths for our stabby friends.
Thus, in most cases as this question describes - this setup probably means mages won't be the ones in control - they'll be working for someone who can command the area. Maybe they could charm the king... but I suspect that wouldn't work out (assassinate the king, and the next guy who can steal the throne puts all the mages under lock and key).
Also, no magical items (sucktastic)? Any magical creatures? Wandering undead, golems? Unicorns, Dragons, Magic-wielding Orges?
This is pretty much the setup for AD&D: Dark Sun, except even they allowed the land to be reconditioned.
I'd recommend against this as it's a pretty horrible world. :P I'd go for some way to recondition the area, improve auras (if they're not already degraded), and something for magical items and more powerful magic - even if you don't want tyrannical mages from your former question.
Also, you need to explain why mages would stay in the magic-poor areas, instead of seeking the source of magic (and being able to do more magic, quicker in the fertile magic farmland of your world). If I were a mage, the first thing I'd do is go seeking a better place.