I'm on EF, so maybe it is different. If it is true that many crew are on 24hrs etc etc (I'm not doubting you) then I agree that is unfair, it's abit like the situation regarding EF CSDs where they have lots of availables and airport SBYs as obviously they are only generally rostered trips with at least 1 sector being on a Boeing (which there are less of them now with the 757 being gradually removed, we have around 5!!) generally only on an airbus if the trip is a mixture of airbus/boeing or if they are called from SBY/Avail due to shortage of PSRs.
I can see what you mean about seniority and of course you are entitled to your opinion. Seniority has it's place as I said before. And of course I know they don't just promote anyone who has high seniority it is performance related aswell. Just, I think all crew who have the required minimum experience should be given the oppurtunity. I heard on the last round of ww promotions, some crew could apply, then were given a letter saying they couldn't due to their seniority, even though they were experienced.