That's a pretty ridiculous qualification IMO because T20 more than other format rewards getting your best players up the order. Your #5/6 batsmen internationally will more often than not bat in the top four domestically because international quality specialist batsmen would be wasted lower than that at domestic level.
FTR I'm not actually saying Bailey is a specialist captain or doesn't deserve his place as a batsman; I just think that particular point was poor. It's almost like saying Anderson is a really good Test batsman because there are few more qualified number elevens; he bats eleven for a reason and it's not because England get the most of his talents when he bats there.
I agree that your best T20 bats will generally come from the Top 4, but there is a completely different role to play in the middle order than there is in the first 4. You don't want your entire batting lineup to be domestic openers; it makes no sense. When Bailey was first selected, I saw multitudes of comments about guys like Rob Quiney missing out. Somehow I don't think picking an aggressive opener to bat 5 or 6 is the best idea.
If I was selecting, I would be looking at a number 5 batsman who could steady the ship if early wickets are lost (while still scoring at a decent rate), rotate the strike and minimise dot balls. That does not correlate to a top 4 domestic batsman, much of the time, who have high boundary percentages but play out more dots (like a Kieswetter, Sehwag, Watson et al). They also have to be adaptable to hit out later on, and take advantage of good starts. In my mind, if Bailey wasn't playing it would probably be the ideal spot for Michael Hussey. He can recover from a poor start or take advantage in the final few overs.
Going in with a batting order of Watson-Warner-Finch-White-Quiney-Wade-Maxwell-Christian (or something similar) would be utterly pointless; you a) run into the law of diminishing returns and b) if you get on top of a line-up like that you end up 6/40 in no time flat because they can't do anything but try and hit their way out of trouble. Each team has a batsman based on their busy-ness and strike rotation playing in some form - Jayawardene, Kohli, Bailey - who can also clear the boundary. You need a balance to the big hitters.
Playing the 6 best openers because they can hit big is not a good strategy, having a balance in the middle order is. Of those balanced players, we have two damn good ones in Mussey and Bailey. George is worth his place at number 5 or 6 as a batsman alone; he performs in that position and adds plenty with his captaincy.
Let's go back to that ODI XI thread we had a while ago, and assume we're taking on the Martians. Very few people didn't select at least one specialist finisher. Perhaps they weren't the best batsmen in history
per se, but they perform in that role better than say, shoehorning Adam Gilchrist into that position when he opened Internationally. Batting at 5 in T20s is similar - it requires a different set of skills to opening, so you don't select the third best opener in that position.