If you have no representation - you have no influence and things simply get imposed.
If you have half representation - you get to start applying some influence and have an affect on all sorts of issues. That influence is, however, limited.
With strong representation you can have a direct affect (as Balpa has done with part time conditions, pay offer from 0% in Oct up to 6% now, FREA more than doubled, and at last the start of some sense on command upgrades, DECs and base transfer policies etc).
None of the above would have happened without formal representation unless you happen to think the half-assed money-grabbing management would have offered it out of the goodness of their own hearts perhaps!!!
What you will never have, even with 100% membership, is any union running the company and simply dictating terms. I cannot think of any airline where this happens - not even the biggest. Maybe it would be a good idea though!!?!