I'm a big believer in learning to read music, just as Simon says. But drumming is also, I believe, flexible. Adaptable. The more experienced you get (and/or familiar with a song) the more free style can happen.
Dr.Vartan, one of my drum teachers, told me that playing the drums is like driving. Let's say you're headed to the library. You think about how fast you need to drive, where the other cars are, and when you should signal that you want to switch lanes, move over, and turn into the parking lot. There are rules to the road, but you've got to think about where you're headed and how fast. Drumming is like that. You've got to keep tempo, move around other instruments if you're playing in a band, and think about where you're headed in your fill, etc. She always encouraged me to read/look at the bar ahead of the one I was playing. (It's harder than it sounds.)
Which is another reason, I guess, to love the drums that much more! In a way, drummers are pirates. I believe, like Simon says, that reading music is an important skill. But it's also important to be flexible, be creative and learn to free style!
I'm what you'd call a "hobbyist" drummer.