I know what you mean, but I still disagree. And I've hoped I explained why, but I will try again:
Please keep in mind that not every tank has high gear, so every tank buster can hurt a ton. They are just doing their thing, so don't expect anything that requires high awareness.
Please keep in mind that not every healer in alliance raids are aware of other parties tanks, mechanics/movement etc., or some are even too fixed on dmg.
This is just a disclaimer, as this is my general experience in alliance raids, so yours might differ.
The most important role in alliance raids is healer, followed by tank, at last place dps.
Let's say we have following situation: (which in my experience is quite common during "content drought", or first week)
Your party's tank is down (for whatever reason, let's call them captain vul stacks), right after some mechanic happens that requires movement, and aoe dmg is incoming. Other party's (and your own) healers are either occupied (rezzing peeps etc.) or dead. 3 tank busters go out, but instead of targeting someone from your party, the 3rd and last tank buster hit the MT, also - just as you suggested. Our MT does not have invul ready, so they are using at least one mitigation CD.
This brings up some questions:
Is the MT aware that one tank is missing and 1-2 CD are not enough?
Is the other (last) tank going to survive the tank buster plus auto hit while the healers are occupied?
And lastly: is the boss going rampage through the alliance?
In my experience this is most likely to happen. This would not happen as long as the last tank buster hits some random dps.
Yes, it sucks, especially when you are top dps. But it's less likely to result in a wipe than with your suggestion implemented.
Alliance raids are what you would call "afk/braindead content". Neither you nor me can expect others looking further than their own shadows. I wish it would be different, but that's just how it is.