I've got a tandem that I bought with a stuck eccentric BB. I suppose eventually I should pull it apart and see if I can get it unstuck. But I don't have my +1 for the tandem so it hasn't been a high priority. Perhaps the problem was just a heavy hand and poor maintenance previously.

Anyway, the belts are not supposed to stretch. So, while one might be constantly tweeking the adjustment of a chain, the belt may be more of the set-it-and-forget-it type, at least in theory. Assuming reasonable QC for the belt production, one may not ever have to adjust the tension even when replacing as long as one sticks with the same brand. With that in mind, the eccentric might not be a bad idea.