I generally oppose those kind of conversions where the gears are left on, for no other reason than it looks like crap and one big appeal of SSFG (c'mon, admit it guys) is the aesthetic quality of the bikes. Even an old rusty Schwinn frame has a certain elegance to it when it's stripped of all of its unnecessary parts and made into a singlespeed or fixed gear.

But here are the mechanical reasons for a proper conversion:

1. Perfectly straight chainline
2. Solid chain tension.
3. An undished rear wheel which is much stronger than a dished wheel from a geared bike
4. Weight
5. My fixed gear chain stays cleaner than my geared bike (I really don't know why).