As owner of a Trans Am for 11 years (with 5 of those years being a completed KITT replica) I can tell you the glass DOES NOT make the car more open to theft. I keep mine locked at all times and parked in a well-lit area in front of my home. It also has an alarm system, which will probably get upgraded.
When I first got my Trans Am, it WAS stolen, however, they didn't break any of the glass to get into it, they just jimmied the door lock. When I got it back, none of the glass was damaged. (Note: This was YEARS ago before the protections were in place, different (worse) neighborhood even...) Besides, Trans Ams have heat-tempered/treated glass. It's not as easy to break as you might think (especially the windshield and hatch, which are thicker). Also, there are upgrades available.
As far as the gas part goes, dude, the guy who said that was playing a car salesman on a TV show! Even real car salesmen say stuff like that to get you to buy from them. I wouldn't take them too seriously, lol. As far as real life, yes, there are cars out there that get better gas mileage, but it comes down to how much YOU want the Trans Am and what you'll willing to do for it and take care of it. Mine has a NEW engine, rebuilt trans, rebuilt exhaust system, new air filter, recently redone ail filter housing, and I'm also adding in a ram air attachment soon. With all these things done, my KITT gets VERY good gas mileage for a 22 year old sports car! Also, most replica owners don't use their KITTs and KARRs as daily drivers (nor do I), which drastically cuts down the fuel costs on them. Even when it was a regular Trans Am it wasn't a daily driver.
In the end, it's all down to how much you love the car and want it, just like anything else. Yeah, it might be costly at times (ESPECIALLY if you're doing a replica, they're 10s of thousands just to make!), but the good totally outweighs the bad. It's all up to you.
