My problem KD is that you seem to have no other reason for why a dead ally is not considered an ally other than "it does weird things to some power." We are working under the assumption that it is because there is nothing that says "while dead you are still an ally"
Without anything to that effect it is clearly a houserule, because the standard definition of the words we are working with would denote that it remains an ally, even while dead.
How so?
Seriously. It is no longer a creature. How can it be an ally? How can it assist you?
Go look up the word ally. It talks about associations and relationships. How can a former ally still have a direct relationship (yes, it could have an indirect relationship such as a will or something)?
1. One that is allied with another, especially by treaty: entered the war as an ally of France.
2. One in helpful association with another:
There is no "helpful association" with the PC if the ally dies.
Is it Fred lying there dead on the ground? Will it be Fred 10 years from now when it is merely bones? No. It's Fred's body. A portion of Fred. It's missing life and soul and thought, the very things that make it Fred.
The body is only a portion of what made Fred, Fred.
If Fred's brain gets sucked out, again, it will not be Fred, even if one could keep the body alive. It would not be a helpful ally at that point.
If the country of France is totally wiped out, then France is no longer an ally of the U.S. The ground might still be there and radioactive, but there are no living people with which to have common association.
So no, looking at it from a strickly English language definition perspective, the word ally doesn't mean a dead person. When Fred is dead, he is no longer your friend. He was your friend, but now he's gone.
Yes, in a world of magic, one could bring Fred back and one could even do so as part of a contract or arrangement. But that doesn't make dead Fred an ally on the ground.
On a side note, this is one of the issues that I have with 4E. For example, elemental powers that only target enemies in an area like Flameburst Armor. The armor is smart enough to target enemies, but not allies. Huh? Ditto for Fires of Judgment, Blade of Fiery Wrath, Fire Shroud, etc.
There are quite a few area powers that only affect allies or only affect enemies. Sword Burst by a Swordmage. It throws force out, but it turns on and off as the swordmage swings his sword in a circle. Huh?
Yes, one can rationalize any of this, but it just seems convenient that PCs can throw fire and lightning around and it is so discriminatory, even in the dark when the PC cannot see. A PC cannot throw Fire Shroud into the dark and accidently hit an ally.