Obvious? I don't see why the torture chamber in my underground, magically-sealed and heavily protected sanctum is that "obvious". Its Infernal aura may be intense, what with 10 years'-worth of ritually sacrificing innocent humans under my belt, but the sanctum is structured to "geographically" limit the physical extent of the aura like the RAW suggests, and I can assure you that no one gets in there except me - well, no one that gets out, not alive, not even as a ghost.
Now, more seriously - playing a Cthonic or Infernal character is a matter of game contract. You can play a full-infernalist in a "shooter" game, and burn for it (or, perhaps, pull it off!). You can play a more subtle infernalist, that hides his Infernal nature very well, without losing a lot of extra power (at least, assuming a demonic familiar). Or you can play a Cthonic character, without ever dipping into serving the Infernal, and do things like scour for demons and ablate them for raw vis. Either way, to be fun it must be fun for other players. If your group is cool with it, any of these concepts can work in the saga. How useful Cthonic magic is for pumping XP in your Arts depends on your saga, but it can be quite useful, which is why I brought it up.
Yeah, that's highly effective. On a personal note, however, I hate ablating. Specifically, I hate the fact that you can truly destroy a demon. An eternal soul that was present at the time of Creation - gone. A powerful demon, perhaps even Satan himself - gone. Because an insignificant mortal wizard learned its True Name and summoned it.
How much more fun is it if such beings cannot be killed! If instead of being killed, they are bound and imprisoned. Or perhaps made to serve their masters. So much more story potential. So much more majesty.
But all that's besides the point. On-point - well, if you've been initiated into summoning and ablating you would probably be better off ratching up your Rego (and Vim) to summon a powerful demon. Regardless, the problem is that you'd need his True Name, as high-Might demons aren't likely to be around to be scoured. And you need high-Might for those bonuses to study from the raw vis. You can still scour the place to gain lots and lots of minor demons and ablate them for lots and lots of raw vis, though, yes. Ironically, that would really cleanse the area from demons.... so altruistic of you...
I note, however, that practically the same result can be obtained by acquiring Second Sight or a high-penetration InVi demon-seeing effect (something you'd be wise to do anyway), and simply marching off to capture (Rego) and kill demons in the local Infernal aura of your choice. Or auras of other Realms, for that matter. I prefer this way, as it leads to more adventures... The "harvest" may be meager when compared to your way, however, and the troupe may not like to throw so many vis-hunt adventures (then again, they could simply be resolved in the background).
While that's true, achieving the level 35-40 maleficia required is quite difficult. If you can somehow pull it off - getting so much vis parva will indeed be ridiculous, and be a definite boost in power 
The set-up required to do so, however, will probably take a lot of time and resources.
If you liked that, check this spell out.
Cheers,
Yair