I have a clip in harness in my car (not RA). I take it out when I am driving on public roads and put it back in for autocross (dirt) and other racing. I really need the harness though because I have a race seat and a normal seatbelt with my seat really doesn't work to good at holding me in. Even if the normal seatbelt was replaced with a strap like on a harness and was pulled tight, the whole left (right for you guys) side of my body is still free to move around and I could slide out from underneath it in an accident (this even happens with 4 point harness). In a crash I might not be able to duck down with a harness but I am much less likely to crash with the harness on. I go racing / track days at least once a month, usually 2 or 3 times, and I wouldn't want to use a normal seatbelt, and I have tried it.

But I really think it has to be up to you. If you don't often go to a circuit then a normal seatbelt is probably best. Really the best option, for any racing, is a full roll cage, race seat, and harness, but that is a little expensive and not so practical for the road.

Aston