In what cases does it pay off to take a scholar character early in the game?
The quick answer is: only if nobody else is doing it.
Scholars + research can be worth a lot of points. You'll start with a scholar and take another one early, then choose research as much as you can afford to. The other big advantage is that if it's not being fought over, you'll save money by choosing the research tile, and you can let your person points slide. If you have to pay to choose the research tile, this advantage goes away.
The disadvantage, of course, is that it doesn't help you gain any resources, so you need to look at the events. If plague is going to kill your scholars, or you're going to run out of building space, you need to handle those crises rather than wasting time on doomed scholars.