So when was the last time 2 likable candidates ran for president? When was the last time you were not voting for the lesser of two evils in a presidential election? I think less of the GOP base might turn out to vote, but I bet a hell of a lot more moderate conservatives and independents do. You probably get a sizable uptick in the number of previously disenfranchised voters heading to the polls.
I see your point here and it is a good one. I don't know what the numbers were in the '08 election but I'd think the turnout was high because the Muslim had set the world afire and seemed very likeable "to most" . You are right though, been a while for 2 good candidates. Using your logic it's really a coin flip, voters could be out in masses or you may not be able to drag them to the polls. Unless you are a dem then someone will drive you to all of every precinct.