I've always used both.  Among other things, if you apply both brakes you're in much better shape should one of the brakes fail suddenly (eg, broken cable, unanticipated wet rim, etc).

But then, I've never pretended to be a racer.

**Added:** It should be noted that, unless you're riding like a maniac (or at least like a BMX rider), 95% (at least) of your braking will occur in situations where there will be no chance at all of a wheel lockup.  In such situations either brake is equally effective, and, if you use both, you spread the wear evenly between them.  This means you can go twice as far before having to worry about replacing a brake pad or a worn-through rim.