Well, I was thinking you're not guaranteed to get a bag if you're DPS—for the following example, I'm going to assume "tank" is the current CTA, though it could easily be replaced with "healer" or (ha!) "dps," so it's only for example clarity here.
My hunter sits in the LFD queue (a task which I am loathe to do once I run out of non-LFD things to do on her) and gets into the group. The group looks like this:
Basanti - Bloodscalp (Hunter)
Appelachiana - Perenolde (Mage)
Alanya - Stormrage (Warlock)
Zenbi - Stormrage (Warrior) - Tank
Xella - Stormrage (Druid) - Healer
Alanya, Zenbi, and Xella queued together, they're in the same guild. When the dungeon is done, everyone gets their normal LFD rewards and goes about their merry way. Against my better judgement, 'Santi requeues and the group looks like this:
Basanti - Bloodscalp (Hunter)
Xellandria - Korgath (Priest)
Xellie - Korgath (Mage)
Xelladin - Korgath (Paladin) - Healer
Delenda - Nesingwary (Death Knight) - Tank
Again, there's a group of three guildies, BUT because the tank is the current Call To Arms and Delenda obviously solo-queued, when the dungeon is done, everyone in the group receives the benefits of Delenda having solo-queued, which 'Santi thanks her for (god only knows what my other alts do, they're stuck-up and only talking to themselves the whole run

In both examples, I've obviously stacked the groups a little bit, and there's a billion ways it can all come out, but in only one of them is the group rewarded for a tank's solo-queue.
The first negative of such a system that comes to my mind is that it discourages partial groups (such as the one from Korgath) from bringing their own CTA, and could potentially have groups intentionally stacking four people, minus the current CTA (healer + 3 dps looking for a pug tank, in this example)... but you can do that in the current system too, and the proposed system is equally discouraging to grouping with friends if you're after a specific reward, so again... I'm not sure how much of a wash it ends up being.