The question has been asked here - why use MPC X-Wings when you have such nice studio scale models available?
Just a guess but I think it was to save time filming fleets of X-Wings together with their wings closed.
Apparently by the time ESB was being made, ILM only had three SS X-Wings left (Red Two, Three and Five). I have never, ever seen a picture of Red Two with its wings closed, so that leaves only two with movable wings.
And we know that they had a lot of trouble with the SS wing mechanisms - with their movements and with keeping them completely closed.
The vast majority of the ILM MPC X-Wings seen here have their wings closed.
Just a guess.
I too am glad to see the MPC X-Wings get more recognition, although I have reservations about how it looks, even with an ILM paint job. I think that the front of the nose (modeled after Red Three) is the nicest part. The rest looks awful - the lower fuselage has wrong angles and curves, and the wings are too small. The cross-section of the aft fuselage is much too tall and thin, and the engine cans too small. But alas, it was the first and only X-Wing kit available to the general public in 1978 . . . then came Estes and others . . .
Interesting theory. I'm just about to start on my ESB Luke Hero MPC and was thinking about doing it with the wings glued shut, metal armature and brass lasers. Only because I don't see why they would have gone through the trouble of having movable wings on a model that would only be seen cruising between hoth, dagobah and bespin.
I think it was down to efficiency in effects shooting. Smaller models mean you don't have to get the camera as far back, so you don't need to mount and light as much blue screen, you don't have to lay as much track, you don't have to program as big a move, and the actual shooting time would be lower too.
I would like to think that they were mostly used for background/long distance shots, where hero details would be lost anyway. (Bespin clouds shot notwithstanding)