I prefer to take each game on its merits and if a player isn't performing he should be brought off. Last night I thought we had a couple of players going through the motions rather than putting everything into the game (Babel and Voronin) and I would have had them both off at half time.
Similarly I think when the team is struggling (which it wasn't last night ) changes should be made as soon as possible, rather than letting things drift in the often vain hope of them turning things around.