I belong to two gyms but am considering quitting both to do exactly what you are thinking, build a home gym.
The cons would be that you would need the space, and it would cost more. I spend about $100 on gyms right now, and I might keep at least one for the classes (spinning, aqua) and courts (basketball, squash, racquetball). So, I think the only realistic reason for joining a gym is if you don't have a free alternative for those. I've currently started playing a lot of soccer and the local parks are free to go to.
At the same time, if you hit up craigslist, I've noticed that you can find some good deals for weights. I'm like you I would probably get a barbell first, but then you've got to have a place to rack it. Then you have to get the weights. By craiglisting everything it looks like I'd spend probably upwards of 500 bucks for everything I'd want with a barbell set. Barbell+500pounds in weight+rack.
So if I quit my gym, within 5 months the costs would be the same, except I can keep them till I'm dead. Plus I have the convenience of working out whenever I want in my own fashion (I like doing circuits myself) where ever I want to do it. I'm thinking that I'd move them inside next to the TV, blast out some bears, then go store them back in the garage.
I do do bodyweight exercises, but squats, RDLs, rows etc just require weight. A lot of it. Having a barbell for me would keep it interesting. I like to do something different almost every work out and it's worked pretty well for me the past two years. I mean, I get bored just doing pullups/chins/situps/pushups everyday. I'd rather do that every few days, with weight in between. Maybe I'm a ADD weight lifter...wouldn't shock me.