I started in powered flying and am now doing gliding as well (solo stage, almost ready for my GPL). I can only support the position of the gliding clubs with regards to tug pilots. There are some profound differences between the glider and the power environment. It helps a lot if you have actual gliding experience, preferably a GPL and some x-country experience. Because as a tug pilot you're fitting in with the glider world, not the other way 'round.