In my experience pilots dont merely tolerate the concept but they can be quite staunch advocates of seniority lists. It would be wrong to give the image that seniority automatically qualifies an individual for promotion to the left hand seat, as the posters have said above its not a straightforward approval, but merely a gauge of their possible fitness for the position. Unlike most careers where aspects of age, experience and attributes are vital pilots are generally experienced according to their hours and the duration of their service with a company. It's not perfect, no system ever could be but in general terms it is quite positive as an objective initial criteria. It doesn't mean that pilots are not assessed for their suitability, it merely means that this assessment will take place with due reference to the time served at the company. It impacts not only pay but bidding preferences for trips, bidding preferences for changing type etc. On the down side there is a lot of luck involved, its all in the timing.
What is interesting is when you have to merge the seniority lists of two companies, Dan Air / BA a classic example, and in the US PanAm / Delta or TWA / American there are lots more where this has been a big issue for the pilots concerned. I would be interested to see how they could merge the pilots without having reference to something as objective as a seniority list? just a thought .