Reach: I'm sure that you realize this, but for the unfamiliar (non-airline types), a union can not save your job if you really screw up or if you to distort or try to hide any facts from the company investigation etc. If you really blunder, a union will not try to stand very close to you.
For the neophytes out there, nothing in this business remains hidden for very long. This world is much smaller than people believe, and faces (along with back-stabbers) are very hard to forget.