I've fenced some 3 minute epee bouts and I like them. It makes the bouts more mentally challenging and less physically. Protecting a 1-0 lead for 4 minutes is actually pretty hard work. The basic upshot is this: The shorter time period puts even more emphasis on the first touch. This demands even more vigilance at the beginning of the bout, since you have less time to make up any careless mistakes made early on.