We've flown with our bikes a number of times. Each time we've taken plastic bags in case the airline insisted on them, but they never have. We put foam pipe lagging on the frame tubes, remove rear derailleurs and so forth, and seen baggage handlers wheeling them about and putting them on the top of stacks of luggage rather than in the middle.
It's true that we've also seen them putting one bike on top of another, and detected the odd scratch afterwards, but so far no serious problems.
If the airline doesn't insist on bikes being boxed, they'll usually be too big to go through regular check-ins, and sometimes too big for the special one they use for surfboards and the like. They'll be swabbed to check for explosives (don't keep them in the same shed you keep your shotgun cartridges in!), and wheeled through the security point where airline staff go airside.
Much better than being dumped on a conveyer.
Wideroe were even happy to carry my recumbent with no packaging whatsoever - outstanding!