There are several reasons to go full frame. The viewfinders of most FF bodies give a larger,clearer viewfinder image which helps in composing every shot taken. A FF sensor is much larger at 864 square millimeters versus 329 square millimeters for a Canon 1.6x APS-C camera. Contrary to popular myth, EF-S lenses of any speed are not really any smaller or lighter than identical speed lenses that will cover FF; look at the 17-55 f/2.8 EF-S--it's big and weighty. And there are NO size savings on telephoto or long zoom lenses. The only small-sensor lnses that save any size or weight are very slow, "kit" lenses.
The biggest reason to go full frame is that the vast majority of Canon and Nikon's lens lineup is designed for use on FF sized imagers. Many lenses do not work well on APS-C. MY favorite example is the 8.5 foot tall field of view to show a six foot tall man and his wife; with FF, using an 85mm lens, you get that field of view from 20 feet. With a 20D-50D Canon, you need to be 34 feel away to get the SAME sized people, in the same height. But, with deep depth of field on the APS-C camera, the background will be very much in-focus with APS-C, while with FF it will be much less in-focus.