Quite a few waltzes (and some non-waltz tunes too) have that pattern where the B part is twice the length of the A (like The Man in the Moon), often because it includes a reprise of the A music. If As and Bs are played twice, it may end up the wrong length for a particular dance, but also if it's played at a session, it can make the tune seem to go on forever, because waltzes are slower and take longer to play than fast tunes. That's one reason why, even at a session where dancing isn't an issue, some players stick to AAB for such waltzes.
Having said that, I'd play the Cumberland Waltz with repeats.

Marje