As I understand it, the Australian situation is a little different from that in the US. Here, all nurses in the public system have the same conditions, whether or not they're members of the union (which is also a professional organisation, leads work shops, publishes a monthly journal...) and there's only one union (with different state branches); I think that in the private system conditions vary from hospital to hospital but not for individual nurses within a hospital.
Being a member of the Australian nurses union (ANF) is tax deductable, means a discounted fee for courses, and includes a range of other benefits (eg $1M indemnity insurance, access to cheaper white goods etc through a multi-union cooperative, legal representation, a free appointment with a lawyer for a will or other issue, discounted movie tickets, journal subscription...)
Because our conditions are fought for by members, I would feel hypocritical if I accepted the benefits without contributing, though I appreciate that others feel differently. I must say that, as another round of enterprsie bargaining comes around I am heartily sick of it, but without the union we wouldn't have the pay, career structure, or ratios.
FYI I am biased - I've been a union rep since 1992