I spoke to one of the senior OJTIs in Maastricht a few months ago on this very point. The conversation went something like this:
Me: "So how many people don't make the grade?"
OJTI: "A few"
Me: "In terms of percentage"
OJTI: "You don't want to know"
Me: "Yes I do, because I want to know why they get chopped"
OJTI: "You really want to know?"
Me: "Yes"
OJTI: "Slightly more than 50%... mostly because they don't do the work".
It's true that the bookwork seems a long way behind you when you start the practical training, but I for one am jolly committed to doing this and wouldn't jeapordize my (potential) career by overlooking to study rather than go out boozing all the time.
I get the impression, though, that there are other reasons for some people getting chopped.