Nothing in The Hobbit says anything about Beorn or skin changers generally needing to be angry to change form. That was a contrivance for the game mechanic. In fact, right after feeding his guests, Beorn goes out for the night to dance with bears (see celebration thread) and check out Gandalf's story. Clearly he did it in Wanderlust stance:
"He had been over the river and right back up into the mountains—from which you can guess that he could travel quickly, in bear’s shape at any rate."
Tolkien, J.R.R.. The Hobbit (Lord of the Rings) (p. 131). Houghton Mifflin Harcourt. Kindle Edition.
I didn't say that, what i meant to say is when it comes to battle and according to lore and nature of beornings it made more sense to me that their anger/wrath generation should be mostly done in man form, because by the time they are changed into the bear form they are already filled of rage and the need to unleash it, not increasing it, until eventually having to cool down. And YES, it fitted perfectly on the class mechanics.
Every time you see Beorn in brink of conflict while in the man form, he seems startled and easily angered, like something inside of him (anger) is growing.. Every time you see him in a battle as a bear he's just rampaging everything around him, but no power in middle earth should last that long, even when it comes to the patriarch of the Beornings, much less us players, the descendants. Of course he can also just "wonderlust" on his days off to dance with other bears, or even taking a pee on trees to mark his territory...