What if you are a 767 pilot and you need to adjust your schedule? Maybe you could not hold the days off you needed, etc. So you what do you do? Drop and/or trade open time. If all open time was fair game and also "pay what you fly" then the 767 open time would be gobbled up and much more difficult to secure. Secondly you would be then "forced" to pick up 757 trips to make it work and that results in a pay cut if you go by your idea. It would be always be a bonus for the 757 guy to fly 767 but could be a forced pay cut for the 767 guy.

While not perfect, I think the concept of separate bid packs only had one legitimate alternative to protect as much WB pay possible, which is a single pay scale and that is not up for a vote.