I do nearly every time I ride while warming up. I just find it easier to get him really extended and moving forward while posting than with me sitting there. :)
That may be the best reason there is for posting in a western saddle. I have gotten in the habit of it since I used to typically ride young horses and sitting the trot tends to inhibit their forward movement. Posting helps me to get out of their way.
I ride dressage primarily, but when I do ride western, I might post for these reasons:
-I'm riding the horse in a english bridle, but a western saddle, to get comfortable with the horse in a more secure saddle, before using an english one. So these are hunt/dressage horses that I work at a fast english pace, so sitting isn't usually the best in the western saddles
-if the horse has a little spazz attack, I might post, because then I don't bounce around, and am able to just think about calming the horse down, and not about staying seated. This of course isn't the case if the horse is trying to buck, or if I'm trying to slow the horse down.
-if I'm just trying to cross the arena quickly, I'll do a fast posting trot.
I definitely don't see a problem in posting in a western saddle. Whenever I ride western, I always do this. Actually in my second western lesson, (long time back) my instructor showed my posting. I think its only in shows you can't post. I actually love the sitting trot ... but it really depends on the horse. My horse has a lovely sitting trot, and I ride English. In an English saddle, I find its more comfortable to sit the trot.
I have seen many different disciplines post for the comfort of thier horse when excersing ect...Trailriding being my forte, I post so not to fatigue my horse and also switch the diagonals that I post to . Good for them, good for me especially to strengthen my muscles.
I post while riding western all the time. I especially do it for the first part of my warm up as it is easier in the horses back, particularly in the winter. I also post when I am wanting a faster more extended trot instead of a jog.