That rear airstairs ARE used, even if there is a jetway, it's undeniably true, regardless if you might have seen it or not. No, it's not used by the airplane crew, but by service people, have a look here:
http://www.airplane-pictures.net/image163699.htmlIt wouldn't be very efficient, and possibly even dangerous if the people doing lavatory or water service, that usually park their vehicle close to the rear of the airplane, would have to walk all the way to the jetway to use the laddder on it.
So please, don't say having a stair on the rear is unrealistic if there's a jetway, because that's not the case.
On top of that, not all airplanes in FSX (in fact, most don't) have the rear door that can be controlled independently, all default airplanes and lots of 3rd party ones have the rear door linked to another door, so we put a rear stair in order not to end up in embarrassing visual situations such seeing a door open in the void, you know those situations that people that don't know better love to comment as being "bugs" (GSX bugs) on youtube...
And, when we'll add service people to GSX (lavatory, water), we need to have a way for them to reach the airplane, since they don't have the luxury of an elevated platform like the catering service, and having them using the jetway ladder, is really too unpredictable, both because not all jetways models have ladders to begin with (and the position/shape changes a lot), but also because it's really not possible to be sure of the actual jetway position when handled by FSX, so we don't really have any other choice (which I believe IS the most realistic one), than let them using the rear stairs.
That's why I believe turning off the rear jetway is wrong, and even if we added an option to turn if off globally, once we'll add the lavatory/water service, if we'll have animated people to come with it, the rear jetway will come up anyway, regardless of that option.