At some point I'd say there's a diminishing return in having that many skaters on the ice. I'd rather keep my goalie in net and prevent the chance of a fluke zone clear turning into a goal than adding an additional skater in a situation where you already outnumber the opposition. I think the big difference between playing 5 on 4 and 5 on 3 is that you can't really pressure the puck carrier and cut off passing lanes effectively in the later. It's mostly the waiting game and shot-blocking, whereas good 5 on 4 PKs tend to allow the forwards to pressure the points.
Oddly enough, I've always felt like 4 on 3 PPs were more dangerous than 5 on 3.