Yeah, Sangakkara gets credit for a phenomenal average since giving up the gloves... but as we saw a couple of weeks ago, when he now takes the gloves, he isn't nearly good enough to be in an all-time side as a keeper. Maybe in his first incarnation as keeper he was good enough, I don't really recall (but I guess he kept to Murali, so he can't have been too bad). But if that's the case, in picking him you're picking a good keeper who averaged 40 with the bat, or a bad keeper who averages 55+.
I'd rather have a great keeper who averaged 32 with the bat back when that was only about 5 or 6 points below a very respectable average for a top order batsman.
And if you want an average keeper who averaged 50 with the bat, there's always Andy Flower.