One nice little perk for two straps over three, is that it "folks" it up a bit, and makes it look far less like I'm dressing up as a wedding piper...
It's the same as the difference between tassels or no tassels on your sporran.

The nice thing is, it would take one hell of a fascist kilt cop to cause a fuss over not having the third strap when you DO go formal.