What kind of rides are you talking about?

Daily riding, I ride from the house.
Saturday morning rides at the bike shop- I ride from the house, but it's only a mile or so. If it was 10 miles, I'd drive like most of the other people do. I'm already slow, and don't want to start out tired when they're not.
Big charity rides- I've ridden from the house a couple of times (10 miles or so), but usually wind up driving.

Usually, it's a combination of time and distance. A metric century is a pretty good workout for me, so I don't necessarily feel the need to go farther. I was considering riding to a ride yesterday, but got to looking and the ride started at 7:30, so I would have needed to ride over in the dark.

One minor problem is if they give you a T-shirt and water bottle and all, and you're on your bike, you've got to tote that stuff around instead of throwing it in the car.