That some one who may not track cars has a different opinion thats fine.
Dave, et.al,
Yep, everyone's entitled to an opinion - even those who don't track their cars. Like you, I have some good track time under my belt although I'm not sure to track the GT when it arrives. I respect those that do... and I suppose I'll end up taking it too after the newness wears off. I've tracked at Laguna Seca, and Buttonwillow and the most times out at Willow Springs (I'm not far from there.)
The issue of the multi-point harness is a relatively simple one. I'm sure with your track time, you are using the belts to get "locked in" a little better as they afford you a better feeling of the car and they also insure your butt stays in one spot. I suppose too, that in many types of collisions, the belts afford more protection that the standard belts. However, in a roll-over (sans roll cage), they are dangerous. Roll-overs are less likely in a GT - agreed, but the GT has limited headroom height and even less so with a helmeted driver, so any roof crushing will likely get transferred to the driver.
The "kids" put them in their cars because they look flashy and make the car seem racier, I guess. You don't see most of these cars at the track. So, if you're a track person, then you need to make the decision based on the pros and cons. If you don't intend to track, I wouldn't recommend the harnesses which, at first glance, may seem safer all the way around. I also agree with Dave that roll-overs at non wheel-to-wheel track events are much less common than a roll-over on the street. The GT, being so low and with such a low CG, is also less likely than most to roll.
Just make an informed decision.