Personally, I don't believe there should be a 'scale' at all. Once you're valid, you're doing exactly the same job as the guy next to you, taking exactly the same responsibilities for people's lives etc..
Some have said it's to prevent people leaving after getting their ticket validated, making them wait for some higher financial reward. Well it's obviously not stopping them leaving from the lack of controllers around here.
How about a training scale, and the when valid(OK do some fudge to make up for delays not your fault etc) straight on to a single wage the same as everyone else doing what you're doing?
(pregnant pause for someone to come out with that "more experience" crap?)