Some schools that track have a lot of flexibility, others have little flexibility.
For instance... Both Minnesota and Davis have tracks (chosen at the end of second year for MN I think, and during third at Davis), but both emphasize flexibility - e.g. you do the small animal track which means you do a required set of advanced courses and clinical rotations geared toward small animal practice, but you've got a lot of elective time to dabble in horses or wildlife or whatever other side interests you might have.
Those are just the two I happen to know about.
I think the main point of tracking is for scheduling, really. They can't possibly hold all the elective courses at times that don't conflict with any other course. So if you put together a bunch of courses that all small animal people should want to take, you can make sure those courses don't conflict. Davis has an "individual" track option as well as a couple mixed tracks like small/food and small/equine, and the students say that there are unavoidable schedule conflicts when you try to mix things up like that. Penn doesn't track, but they *do* try to schedule courses for minimal conflict within the traditional categories (e.g. all the courses a small animal person would take, all the courses an equine person would take). And the students there reported that there were schedule conflicts if you tried to color too far outside the lines of those categories. So essentially... they're tracking too. I'm guessing everywhere has this problem to some extent.