Let's see.... at my company the union successfully fought to get the jobs back of at least 4 pilots this year alone. They also defended pilots after being involved in a/c mishaps. I like having career insurance, personally.
Other good point:
Let's say your non-union carrier falls on hard times. The company decides they need to get rid of one aircraft type. Rather than retrain everyone in seniority order, they're just going to furlough everyone on the aircraft they're getting rid of, even if they're senior to the other ones. Because they don't want to pay training costs. You can laugh, but it happened at Midway Airlines. Oh, wait, I forgot they might not even furlough you they just might fire you and tell you when you come back, you start at 1st year pay.
Unions are a necessary evil in this industry. You can make them less evil by personally holding your MEC and reps accountable for carrying out your wishes.