I think it depends entirely on the person. You can work 100% on behalf of Union members and then go and work 100% for an airline; the airline work does not negate what was done before. In some ways having been on both sides helps both sides - there is understanding of the processes and it cuts out quite a lot of the BS. Willie Walsh is an example: he was firm but fair and when BA38 landed short at LHR he was the first person to stand up and say they did a very good job. Someone without his pilot/union experience would likely have been very hesitant to do this.
For someone who is technically and personally very competent with maybe a side order of ambition, there is nowhere to go once you’ve got into the upper echelons of a union, so naturally they gravitate towards the potentially more rewarding jobs in management. That’s fine by me as I would rather have them working me for a certain period then moving on to greater things than not at all.