Someone has not tried going into and finishing the Arena while doing AoMr. Try it (on medium for sure it's damn hard since the highest level you can possibly get by then is level 3, and even then those cacos are effing hard to kill without being blown to pieces) and you'll see that you don't *always* go into special levels; sometimes you do say to yourself that it's just not worth the risk (and with the Arena, if you don't finish at least the second wave [where cacos are practically guaranteed to appear] you get only XP as a reward, which won't be much of a trade if you lose a kit/armor to the stuff that shows up). Or perhaps better yet, the CoS without a shotgun/lots of speed shooting, or the Spider's Lair without a phase device (though those teleporters are probably enough to give you the room needed to take it safely).
Not that this might not still be a good idea, but I don't think we necessarily need this to happen. Though, one interesting question does come out of this: how many special levels are relatively simple to clean out entirely?
* Arena - not what I'd call simple due to the cacos and three waves you need to deal with before you get a really worthwhile reward, and without a chaingun or cs those cacos are scary, especially on lower difficulty levels when you'll have at most 2 or 3 levels worth of perks
* CC - definitely, the zerk packs make even the cacos into cannon fodder
* Wall - maybe, but only if you have the RL, plenty of ammo, and don't end up with too many Barons coming in range too quickly
* Armory - definitely, toughest enemy here is the demon, which by the time you get here ought to be a piece of cake
* Spider's Lair - not really; teleporting might make it easier but you'll still likely have to deal with multiple arachs in view, which is very likely to get you slain once they wake up
* City of Skulls - maybe, but without a quick trigger finger or a cs/ds, you'll get overrun with skulls pretty fast and be nibbled to death.. by the time you get here, you'll probably have one of these, though
* Mortuary - not truly simple to clear even if you can reach the AVs before they raise an army; those flame pillars *hurt* and every time they raise something it's another distraction
That makes, what? Two places you're guaranteed to be able to trash and several more that depend on what you're carrying/you've taken as your traits? Of those two, only the armory IMO is really worth visiting for the item rewards (berserk is nice, but only really useful if you can find things to kill before it goes away, and if you have no RL you can't get the speed mod).