Personally speaking, I would not want one. I work for my employer, but my union give me the facility to collectively negotiate with them and this is because of the law.
Until I see employers move out from the umbrella of legislation, then a union member I will remain.
Just take DH for example. This was such a high profile case in this dispute, if what BA and pprune says is true, then it was a cut and dry case. Just think if BA had offered binding arbitration as its first option.