Mine are not so often dirty either. I'd imagine that brushing them might make them stay clean longer? Anyway, I'd give them a bath anyday over traveling to BB, because at least the bathe would build relation etc. while the travel seems quite wasted, unless I'd also take the time to hang out with someone in the park there.
Added: I really miss a shabby bathtub for my garden or backyard. It would be more fun giving the dog a bath if taking place in a corner of the garden dedicated to pet care :) But the indoor tubs look totally out of place if put outside ...