Surely if you was going to buy one you would arrange the same with a group of friends so you all had one :S?
It depends how you plan your visits, as has been said, the park have given you ample time to book your ticket at £5 in advance, this helps the park get an idea of how many people will be visiting which I expect is a great help to them, with the fastback being worth £25 you only actually have to have 3 spur of the moment trips to make this worth it.
If your someone who plans trips in advance then its worth paying the fiver now and the bolt on isn't worth it. Just because its not value for money for you don't mean it isn't for a wider audience!