The same could be said the other way round.
Having seen a lot more of Sorensen this season than Foster (obviously), I can think of one big mistake that Tommy has made resulting in a goal (Boro away), and can think of two for Foster just off the top of my head (Robinson's goal for Spurs, Vassell's goal for Man City). If I can think of those, I'm fairly sure there must be more.
Every goalkeeper makes mistakes.
Sorenson is lucky to be playing in front of a far better defence, and they often bail him out. The amount of goals we concede fomr set pieces is definetly higher because of the fact hes in goal for me. Mellberg, and as a result, Laursen, do NOT trust him in anything.
Hes made loads of mistakes this season, not necceserily resulting in a goal, off the top of my head one being his kick away to westham straight to dino neck. Also off the top of my head Milners shot away at newcastle when he sort of just watched it go in over his head.
Anyway yes, I do take your point keepers make mistakes, but Id rather have Foster as he'll sort out our back 4 from set plays, and keep us in games with saves that are not meant to be made.
Hes one of the best keepers ive ever seen in a 1-on-1 situation. He comes flying out before the forward sets themselves and so often smothers the ball.