For me there aren't really any cons, though others might disagree. Depending on your area, the annual membership can be from $90 - $300, average being around $125, I think. If you work a lot, do a lot of independant contracting, studio work, etc, being a union member is a no brainer. Being a member of the union opens you up to a lot of gigs, too... good for you frelancers. You're given a lot of protection you wouldn't otherwise have if you went it on your own. A manager stiffs you on your contract.. the union has lawyers willing to fight for you. The airline tries to charge you fees they said they wouldn't, your union rep should be able to take care of that(as they have for me twice!). Homeowner's insurance doesn't want to give you more than $250 for your '59 P-bass that perished in a fire because they see a "P-bass" on eBay for $99.. the union is there for you.
You are expected to show the utmost integrity as a musician... take yourself seriously... this might mean turning down some gigs that aren't up to snuff. I can understand how this might be a problem for some people, but keep in mind- that's what makes the union strong- MEMBER INTEGRITY(someone ought to start a new thread, I could rant on this something awful!).
There are a lot of "underground" music networks that are union-like, and some of those might work better for some people. The AFM is probably the largest, the most diverse, and most well respected. And the monthly magazine's pretty good... a nice median between the monthly academic music(Downbeat, Jazziz) rags and the monthly newstand music rags(Guitar Player, etc)... it's about "musicians" rather than gear, rock stars, gear, rock stars, gear, rockstars or... gear and rock stars.
(that said, I haven't renewed my union membership since moving to NY!

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