I don't spent tokens on them because that seems like a bad trade, but I do spend units on them, specifically when I am one trophy away from promotion. In those situations I feel it is worth it. First, because my units come from the arena so I'm not spending cash on them, and second the frustration I avoid not losing a trophy when I'm one away from promotion is worth it all by itself.On an objective reward cost/benefit basis I'm not 100% sure it is worth it, but purely in terms of improving my overall experience it is completely worth it for me. YMMV.