Ok, here's my two cents...
Unions are complicated. In many ways they do achieve what they were founded to accomplish: to protect and support the rights of their members. Unions are mostly protective organizations. They protect the rights, safety, creativity, and finances of their members. Because of this, a union often is perceived to protect its weakest members most effectively. That stagehand who's been in IATSE for 50 years, but doesn't have good strength left to pull a rope - his rights to continue working in his chosen profession are guaranteed. On the other hand, union regulations balance the diva performers who believe that things like call times don't apply to them. It works both ways - unions protect the original creative work (demanding that actors don't change script lines on a whim), the onstage and backstage talent (in terms of physical safety and fair wages), and the producing organization (by setting clear and mutually agreed guidelines by which any complaint/incident can be measured).
Since generally only larger companies use unions, they cannot prevent the local community theatre tyrant Artistic Director from holding 16-hour long rehearsals. Most people starting out in an industry will have to spend some time "paying dues" by long hours for pitiful pay before they actually Pay Dues to the Union for protection from such practices. On the other hand, because the larger, more influential companies are usually unionized, they do set an effective industry standard. But with this oversight and protection comes bureaucracy, paperwork, accounting, and other pesky things not directly involving the onstage performances.
To join or not to join, in my estimation, depends on your career goals. If you're going to be an SM forever, you might eventually want things like health care and a retirement plan. A union will help you reach those goals, and you won't have to take a part time gig at Starbucks to achieve them. If you're not a career SM and you just love pouring your heart into the local theatre after your 9-to-5, and you can't imagine any oversight organization telling you that you couldn't hold tech sessions till 4am without sending your beloved company belly-up, then a union is not for you. If you don't see yourself SMing for life and want to keep your options open as you gig, maybe a union is not for you. If you're not sure where you'll be in 10 years, but today you want to be the best SM doing the best shows at the best theatres, you might want to join a union no only to enhance your professionalism but also to increase your employability at higher levels. Then again, you might not.
Me: I'm not a union member and never have been. But I am trying to guide my career towards upper management in opera. I see my ultimate place as being in the Board Room and behind an office desk. (Never thought I'd say that!) I have worked closely with many performing arts unions, including IATSE, AGMA, AEA, USA, and AFM. I issue AGMA contracts in my current job, and have actively planned and participated in IATSE and AFM negotiations in past positions. I'm something of a Democratic-Socialist, so I fully support the existence and goals of union organizations just because of personal philosophy. They do put certain demands on business, but they are not bad business. And if any of this ridiculously long post is useful to you, you can certainly quote me - PM me if you'd like more info on the issue or my opinionated self.