The reason for constant height, in my mind, is because it makes movies seem bigger.
If you have a 16:9 screen, then you can only utilize it for, mainly, TV and 16:9 movies. Now, probably 80% of blu rays (if not more) are in 2:40, which means for 80% of your movies watched, on a 16:9 screen, you will have black bars on top and bottom.
I ended up going with a 2:40 screen because the movies seem bigger than regular viewing. When watching TV, most programs are 16:9, so then, when I pop in a movie, it just seems like it is a bigger experience, which in fact, it is.
If, however, I went with constant width (16:9 screen), then the movies would in fact seem smaller than regular viewing, because, it is (black bars on top and bottom).
It really boils down to this:
If viewing movies mainly, then do the 2:40 screen. If movies will rarely be watched, and the screen is mainly for sports and TV, then do 16:9.