Some DL planes (MD-88s come to mind) have the trays in the seatbacks on exit rows. The trays have to have an extra extension in their arms to reach the seat. That kind of arrangement is slightly less stable.
If you are a "big guy", the fixed armrest may be constricting, but if you are sitting
next to a big guy, the fixed armrest will guarantee that they won't encroach on your space.