The point being is tanking the bats requires healers to single target heal both tanks. With someone kiting you're never getting hit by the bats, this allows the healers to focus the majority of their attention on the raid. The fight is a lot easier when quakes are done properly, and quakes being done properly require the healers to heal people with crystal shells to/near maximum capacity. This is a lot easier to do when you have a single tank to worry about.
With one person dealing with the bats the raid need only focus on killing the turtles. Bat and turtle spawns USUALLY line up at the same time as the fight goes on and removing the bats from the fight allows people to focus on the most difficult part of the encounter, killing the turtles. The bats just don't roll over dead, they still have a decent chunk of health and will take away time from killing the turtles. Turtles being stable makes healing and DPS a lot easier and as I mentioned earlier have a surplus of bats up gives some damage dealers in you raid more survival/single target damage by getting fed procs from them.
The best thing about howling blast is you can pick them up in the air, in one tight clump more often than not. When we tried ground based AoEs they provided mixed results as it takes quite awhile for the bats to actually reach the ground (but they can still hit people when they are almost to the ground).
Brewmaster does have more room for error in not dying but DKs still have the easiest time picking them up. We had something like 15 attempts before we killed the boss on heroic and the bats were never a problem to pick up. The stun from quake is only a couple seconds and as long as you keep a 15-20 yard distance the bats won't reach you.
Another thing to keep in mind is Brewmaster AoE is just a snare that doesn't do damage at all. It's much more spammable than HB though. On our kill I did amazing DPS up front, contributed heavily to the first three turtles and still did good damage to turtle spawns later in the fight (because howling blast is still going to hit a lot of them). Howling blast spam with warlocks getting procs (embers/shards) does eventually kill some bats, Brewmaster kiting probably isn't going to kill bats. This can be a problem for some people with lower end machines as they will start to experience FPS drops when you go over 20 bats. On our kill we never had more than 20 bats up a time.
Like I said there are a lot of ways to do this boss. You can single tank it with three healers, two tanks with three healers, two tanks with two healers etc. Kiting is just another option to throw into the mix. I main my DK both frost/blood with my alt being a disc priest. For us it was either 3 healing with my alt or two healing with me kiting. It was an incredibly fast kill on our part and it made everybody else have an incredibly easy job. Any encounter IMO where you can offload all the difficulty on a single person is something people should take advantage of, on the grounds that the person can perform that duty. If you kited heroic deathwhisper adds in ICC (which is much more difficult) as a frost DK, this is right your alley.