Its up the individual whether they resign or not. Most stay in because in the event of an 'issue' with the company they will have union back up and at the end of the day they are entrusted with the negotiations regarding pay and conditions.
I'm no fan of BASSA but its the memberships union and nobody elses.
I'm not convinced the membership would stay as high if members had to pay their dues anually with cash rather than have it deducted by check off.