More part timers would join the union if they didn't have to pay dues......drivers could pay a lil more making $80-90 grand a yr.......More union members=stronger union.
Imagine the scenario of part-timers being allowed into the union without paying dues, while full-timers are forced to join (in many states) and have to pay monthly dues.
I envision part-timers quickly becoming "second-rate Teamsters" in this scenario-- PTers are there to bolster the numbers but, as they don't contribute to the union, will face lackluster (to be polite) union representation at best.
A slightly similar set-up would be, in a criminal law situation, choosing between hiring a public defender or a private attorney. Public defenders are dirt cheap ($50 down in my state's county, I believe), and lawyers'll end up costing you at least $5 grand in the long run. Both promise to look after your best interests, yet private attorneys seem much more successful than public defenders at obtaining a result that reflects their client's best interest. The biggest, most important difference? How much money you give them/they earn. Sure, time and caseloads factor in too, but it all boils down to the almighty dollar in the end.
$0.02
P.S. I don't mind paying dues (retroactive initiation fees are a bitch, though) but I do wish we paid a flat percentage as opposed to multipliers of our hourly rates.