Here we go!
In my experience there was certainly a considerable proportion of people who went off to ETPS because they didn't quite cut it on the front line on their first or second tour. These people in general disappeared into the TP world, never to be seen again. There were also a lot of people who were very competent and either stayed in the TP world or returned to the front line (with varying degrees of success!)
It was often extremely frustrating to have restrictions put upon you on day to day Sqn ops by people who didn't have the ability to cut it in the first place!