As a previously overweight (probably obese for my then height of 5"7") person, I can personally say that it was going to the gym and seeing the people in better shape than me that was motivating.
I understand the objective; to eliminate the intimidation that exists (it definitely does) for newcomers/people with a long road ahead of them. However, it doesn't change the requirements.
If you're not willing to work at it, you're not going to get anywhere. If you're not willing to swallow your pride for a while and go to the gym, then you obviously haven't reached that level of desire for change.
The frustrating part for me was the guys that looked down on you/made you feel like shit for even trying to pick up that weight/use that rowing machine. It was difficult to stay focused knowing the super attractive-super asshole (why must they always be good-looking and stupid!) across the way was laughing at you to his buddies.
I currently go to a gym that is more of a support team than an independent gym. There are gym rats there too, but they're HELPFUL. POSITIVE. CONSTRUCTIVE. I've come a long way from being the fat kid that gets laughed at but still have a ways to go, and I'm glad I have a healthy productive environment to do that in.
(MAN do I RAMBLE, sorry guys
Point of all this: I don't think it's going to make a difference to only allow heavy people into a gym. It WOULD make a difference if you could somehow screen out the egocentric douchebags (then again, my hatred for them did fuel my workouts sometimes). The tools necessary for change come from within.