I think somewere in the middle - have been at camps were staff were hired for a specfic thing who did want to spend some time in the units and with others who really didnt want to spend time in units - and were very had to work with when they were put in a unit - so maybe giving people with certifications the choice of what they do!
ideally (not necessarily what actually happens) offering certifaction training in the week before pre-camp starts for free or charging for it?