I'm posting this as a 15 year Operating Engineers union member, so there may be a little bias

The union makes perfect sense for my industry. The empoyment is often seasonal, and companies grow and shrink quite rapidly depending on what contracts they have. Even the best employers, the biggest companies in the construction trades go through rough spells, sometimes missing on several multi-million dollar bids in a row by only a few thousand bucks.
They can't keep all their skilled workers employed under those circumstances.
The Union Hall knows what I and my fellow workers are qualified to do, and when an employer calls, they get a good operator with one call. If they don't, or otherwise butt heads with the guy, they are fired. Simple as that. There is no such thing as seniority in my trade, and if you don't cut it, you are gone. Sure, employers honor seniority on it's own merits, because the person they have had there for several years is also likely to be the most knowledgeable and productive.
In the construction trades, being non-union means having a 401K or retirement plan from every different company you have worked for and having breaks in your medical coverage in the slow times between jobs.
I have one pension plan, and one medical plan. I don't have to mess around re-applying for medical coverage, and being paranoid without coverage during the waiting period.
I get 1 month of coverage for each 100 hours of work, and can bank up to 7 months of coverage that way.
I watch my one retirement grow, even though I have had dozens of emloyers over the years.
The employer gets a trained and qualified worker, who has pride in what he does. Someone had proposed that with no pay incentive, there is no reason to do better. That is the biggest load of hooie!!! There is tremendous pride in my trade, and a desire to be one of those operators that the others can watch and say, " Man, how does he do that? he makes it look so easy...."
You work alongside the good ones, watching and learning what little things they do to be the best. You do your best. When the slow times come, YOU will be less likely to get laid off, believe me. That is plenty of incentive.
In addition to that, the very best and productive workers can privately negotiate for a premium over and above the wage package. If you make the company that much more money, you are well worth it.