Rooney seems like the type of player that is almost completely ruled by his emotions, a bit like Paul Gascoigne. He's capable of playing with the best (or was), but he seems to allow himself to get disheartened and frustrated far too easily. His temperament is his biggest flaw.
I think that's why Roy was reluctant to bench him, because Rooney would implode and Roy presumably thinks we will really need Rooney at some stage, if he is on form. If he keeps turning in poor performances though, of course Roy should make the brave decision of benching him.