Notre Dame [correction: University of North Dakota] lawprof Eric Johnson notes that many law professors seem to be former high school debaters and suggests that this may be a common phenomenon. For what it's worth, I'm a former high school debater myself (1 year of policy, 3 years of L-D). My anecdotal impression is that there is a high representation of ex-debaters not only among law professors but also among other social science and humanities scholars.
Why do so many former debaters end up in academia? For the most part, it's probably correlation rather than cause. Both debate and academia tend to attract highly intellectual people with an interest in politics, law and related subjects. But there may be a causal connection as well. Debate played an important role in making me comfortable with public speaking - which turned out to be very valuable in later years. And for reasons I discussed here, it also led me to work much harder in school, thereby rescuing my floundering academic record and enabling me to get into an elite college (without which I might never have made it to Yale Law School later). These experiences may be atypical. But I suspect they parallel those of at least some other debaters-turned-academics.
On the other hand, I have to say that at my school, unlike Johnson's, debaters didn't usually hang out with "the cheerleaders and football players." Whether that helped my later career as a lawprof or not is hard to say. On balance, I continue to believe that the real enemies of debaters and other highs school "nerds" were the popular crowd, not the much-maligned jocks.