There's some serious overrating of AWB in this thread, when he's been a weak link for us for a while.
He's very good in specific circumstances - when we take a low block approach and have to do a lot of defending, against a team that doesn't put the ball into the box aerially, he can be brilliant as he's very difficult to dribble past.
Unfortunately, outside of that specific set of circumstances, his shortcomings hold us back; he's poor against teams that put the ball into the box, as he's not very good in the air, nor is he great at marking. He's poor against teams that sit deep and hit us on the counter, as he's quite lazy when tracking back, and offers almost nothing going forward. He's poor against teams that press high, as while he's a decent dribbler his passing leaves too much to be desired, and his concentration levels are often found wanting.
Most importantly though he just isn't a good fit for the way we want to play, with a high defensive line and players comfortable playing out from the back, as that approach highlights his weaknesses and doesn't allow him to use his main (and almost only) strength.