I really just don't think it makes sense to say that someone who could eventually be a top-six player should show that he is a capable fourth line player. Some guys who have the offensive talent to be a top-six forward simply aren't going to thrive in an "energy" role, and I can't see what's so wrong with that. Imagine a guy like Sykora on a fourth/energy line. He wouldn't be a very effective player. At all.
I hate the idea that a guy has to prove himself on an NHL third/fourth line before he gets top-six minutes. Its wrong to suggest that the progression of a player ought to be 4-3-2-1 in the minors and then 4-3-2-1 in the bigs, certain players' skillsets will only be put to good use if they are playing with other talented players and they are allowed to be creative.
I think what they're looking for is the effort. If they were to play Pesonen on a bottom line, they'd want to see him work hard, play the system as instructed, play defense, etc. Therrien demands that from all his players. It's why Sykora and Satan both get demoted from time to time. It's why Dupuis and Talbot get opportunities on the top two lines. Obviously, Pesonen won't put up big offensive numbers without playing with skilled players. But he's not going to get that chance if he isn't willing to play hard all the time in all areas. Pesonen can't control where the coaches play him, but he can control the effort he puts in, and if he puts it in, eventually he'll get his chance in a bigger role. It's all on him.