I've seen lots of people that run both. You can tuck the harnesses out of the way when you don't need them. A harness is hard to live with on the street, you can't lean forward! As a matter of fact, if you are correctly wearing a 5-point harness, you can't move at all.
I tend to see things the opposite of graewulf on this subject. A harness system is more appropriate for things that involve speed. If you play on really big hills where MULTIPLE rollovers are possible then you need the harness, but rolling off a 4 foot rock ledge at 5mph doesn't really call for a safety-dated NHRA-certified harness system.
If you are going to install a harness, you need to get yourself an NHRA rulebook or look up the rules concerning the correct placement of attachment points. If you install it wrong, the harness itself will hurt you in a crash. Placing the mounts for the shoulder harness at the wrong height or angle can cause severe spine compression followed by permanent paralysis in a frontal impact.
Reasonable prices on harnesses can be found at the dragstrip. In order to remain legal, racers have to replace or recertify when the date tag runs out. To them that harness is junk, to a fourwheeler it's as good as new as long as the webbing isn't torn or chaffed. New ones aren't that expensive though, about $130-$140 for two Simpson five-points from Summit, I think. Just make sure they are SFI approved.
