I view union work pay like LCA/Inst. pay.
You don't want to pay so much extra that guys who only care about the $$$$ take the jobs and do a cruddy job and crowd out the pilots who WANT to do the job and have passion for the work.
Yet, you want to give the guys who have a passion for the job and do the extra work involved to do the best work possible bonus $$$ for their efforts.
It's a catch-22.