There is no compelling reason to join AUK other than it is the decent thing to do if you intend to ride AUK validated event.
Picking up Martin's point, when I run events (planning to do this in 2010) I'll be offering something to benefit AUK members. Possibly vouchers to offset the cost of tea/food at the control cafes or free food/drink at the start/finish. Or maybe even a lower entry fee.
In other words, non members will subsidise members in some way.