The think with ABdV as a 'keeper is that he clearly wasn't much of a wicketkeeper, per se, rather he's an absolute gun who can do anything and is a ridiculously talented freak. The big criticism levelled at him, from a "Why I don't have ABdV keep in an ATG XI" scenario, is that he is so, so reliant on his athleticism and freakish abilities to catch the ball -- he's not much in the way of a technical wicketkeeper with his footwork. Sure, it looks impressive when he dives across in front of first slip to take a one-handed catch, but someone like Peter Nevill probably takes that in a completely regulation way, on his feet, with his gloves at his inside hip (i.e. the technically correct way).
It's not dissimilar to the criticism levelled at Matthew Wade -- the big difference between AB and Wade is that AB has an amazing pair of hands while Wade just, well, doesn't.
AB has never been more than solid up to the stumps to spinners, because that's far more demanding on your wicketkeeping technique, and requires far less diving around. If you're happy taking that in an ATG XI, that's fine -- go right ahead, I won't tell you you're wrong -- but for others, they want the technical aspect of wicketkeeping better addressed without losing much batting compared to ABdV. And for that, arguably, the ideal balance is Gilchrist.