I am eligible to join the VRCT, but why should I join the "voluntary" register and not just wait to join the "compulsory" one?

The reasons I hear/read to join the VRCT are as follows:
1. Join if you're eligible & you'll move onto the compulsory register, it will be more hassle if you don't! - Why? If I'm eligible now and these are the criteria the HPC will use, I'll be eligible to join the compulsory register. (10.00 VRCT fee saved!)

2. It will involve more work to join later on. - My wife's profession became registered recently, she had to complete just one form (sent to her by the regulating body - she did not have to complete a CV, send a J.D and get her completed form signed by two seperate individuals). Also her employers pay!!!

3. The cost will rocket! - Can you assure me that, should I pay the VRCT 10.00, the regulating body won't come and ask me for the difference when they agree a fee?

I look forward to some positive replies.