On tours, that's where I carry spokes. Wrapped in a plastic bag with a wad of rubber bands as a plug to keep it in place.
With two of us on tour, I want to carry 8-12 extra spokes, and that many just doesn't fit nicely anywhere else on the bike. Luckily, I've only had to take 'em out a few times.... Messing with the seat is a pain, but considering that fixing a broken spoke is at the bottom of my list of fun things to do, it's just noise.
-- Mark