The frame makes the cab heavy and a great base support for a large Monitor. Real arcade monitors are heavy and a frame is great support. That being said a flat panel will not need one.
Now the flip side to all of this (and this is a personal preference) is I like a heavy cab. I have a few 300lb buddies that love their fighting games. When they really get into it, my heavy cab does not budge as they yank the joysticks. I also built it knowing that I was not going to move the cab around.