Wonker you've summed it up there. BALPA does more than just negotiate T&C, it provides support and protection amongst other things. For those who don't want that they are carried along with the T&C negotiated. It's up to the individual to choose if the price of membership is worth the subs to them. Morals apart - if you're OK with someone else paying to which you receive a benefit without contributing.
Gorter I'm not sure you're correct in believing that a company will negotiate with an individual. I think that most will only negotiate with the agreed union if there is one.