I've never worked in a Union but have worked along side Union people and yeah seniority can make it hard to trim the worthless. My dad was forced into the SPEEA union against his will on their last contract. Until then he was never in a Union either.
The plus that still exists is safety in numbers in an employers market. The flip side of the coin for un-organized labor is that many corp's will walk all over their employees in an employers market. Then the pendulum swings to an employees market, people walk and the execs wonder why. The pendulum might be starting to swing.