the p&t bag is very expensive, but gives good protection - very well padded, and lets you wheel the buggy along in the bag too.
I wouldn't put my p&t on a plane withut putting it in a bag - too many straps/clips to break/lose and the handle is prone to damage too.
we have managed to get airlines to stow our p&t on board efore now, as once it is in it's bag, it is a similar size to a suit carrier, and dh has wangled it onboard (rather than waiting for it at baggage reclaim) so form that perspective it was worth every penny (dd1 is disabled, so having the buggy immediately was a godsend)