How exactly would you measure their performance? You could end up paying someone that doesn't do anything, but as far as you know they've had no reason to.
Bad idea.
They're the same as the rest of the users except for the fact that they are responsible for the content of the forums (more or less). It's an honor and a privilege to be a moderator and it is completely voluntary.
So no, I don't think moderators should be paid. They are (generally) the most fair/respected/reliable/good members of a community and are picked from the community for that reason. But it is a voluntary thing. They are under no obligations or contract; nor do they work for Bungie.
On the other hand, the Web Devs (like Achronos) should be paid because they work for Bungie and are responsible for the day-to-day workings of the site.