I think the biggest thing is the disk space savings. The only con is if you set reservations @ 100% you lose your memory overcomittment. I've seen the 50% number floated around the forums a couple of times. That looked to be what most people thought was the sweet spot.
The swap file is sized by memory - memory reservation so if you have a 2 GB machine and reserve 1 GB you will only have a 1 GB swap file.