Pin gap is a factor to remember when you're selecting both the size of the pins and the amount of pins in the sight.
If the bow is up in the 300 fps range, you might find that you only require a three or a four pin sight to reach out to 45 or 50 yards (beyond that I'd really not recommend shooting at an animal - even that is stretching it).
Faster than 300 fps you should only need three pins. The five pin sights will make the pin-gap too small and you'll get all your pins bunched together so they will appear as if they're touching and they will look like just a big clump of color - as extremearchy has said.
My hunting bow puts out 260 fps with arrows that weigh in at 370 - 365 grains each (including the broadhead). I use a Spot Hogg Real Deal 5-pin sight with .019 pins. Even at that slower speed my pins are all clustered together in the upper portion of the sight. They are just about too close together.
IMHO you need at least some pin-gap so that you can acquire the correct pin real fast when you're in an excitable hunting situation and don't have a lot of time to make the shot.