I have always been a union member during my years as an employee..
Among other things they are there to assist with the power imbalance that can exist between employer & employee in any negotiation.. They champion workplace safety & conditions, all of which often seem to slide when left up to the employer..
For the most part, I believe they are a worthwhile organisation & I encourage people to become members because I believe they do more good than bad, & are a necessary counter to corporate neglect & greed...
As to whether they may be of a personal benefit to you I can't say, but that may not be the best or only reason to join one IMO
Well it's not Illegal or unlawful...
From my limited understanding an employee earning above that threshold can still bring a case of unfair dismissal to the FWC as long as that income package forms part of an award or enterprise agreement.
In other words, if you have a private agreement or contract over that threshold you will be excluded from using the FWC to fight your claim.
I can only speculate that being a Government funded body, that like legal aid means test, there is a cut off point beyond which they consider you may have enough resources to fight your claim in the courts using private legal representation.
As an example a breach of contract, or evidence of discrimination could be grounds to take an organisation or company to court.