@Cofty,
Over on the JW apologist blog "Opposers Dismythed" (ran in part by our own in-house apologist Bobby), someone commented with a point similar to the one you were making on the OP and got a response from the JWs. Here was the comment:
The Israelite were in some cases allowed to eat animals that were found dead, animals that obviously had not been bled (Lev.11:38-40). Also, if an Israelite brought blood from a still living animal to the altar, said blood would have no sacrificial value, only blood from a dead animal would become sacred. In light of this, what would your response be to those that say that blood from an organism that has not died, as is the case with medical blood donated by a living patient, is not the same as the blood from a life that has perished?
and here was the response from the JW apologist:
Anonymous:
The law requiring bleeding an animal before eating it is absolute (Leviticus 17:10-14) and could only mean this would be a domestic either killed by a wild animal, thus bled, or killed by an eviscerating accident, thus bled. However, another law forbids the eating of animals torn by wild animals. (Exodus 22:31; Deuteronomy 14:21) The laws in Leviticus 11:39-40 and 17:5 show that there is no punishment for doing so, but they must be seen as “unclean” and are not allowed to eat the sacrifices while they are unclean. (Leviticus 7:21; 22:1-8) But it is abundantly clear that such things were not to be eaten along with the blood.
Yes, the sacrifice must die in order for the atoning blood to have any value, which further highlights the ridiculousness of comparing transfused blood to Christ’s blood. (Heb 10:29) However, that does not in any way undermine the law that says that blood must be poured out regardless of whether it is used in sacrifice or not. (Genesis 9:4; Leviticus 3:17; 7:26, 27; 4:25, 30, 34; 5:9; 17:10-14; 19:26; Numbers 19:5; Deuteronomy 12:16, 23, 27; 15:23; Acts 15:20, 29; 21:25) In fact, it is one of the most oft-repeated laws in the Bible. (Leviticus 8:15; 9:9; 1 Samuel 14:32-34; 2 Samuel 23:16, 17; 1 Chronicles 11:18, 19; Eze 24:7; 33:25) Add in all the mentions of the use of blood for atonement and cleansing (I will not enumerate them here,) and the value of blood becomes the most important subject outside the Shema.
But to answer your question clearly, David poured out the water his men brought to him from the cistern that he expressed raving for, calling it the blood of his men, because his men risked their lives, yet not one of them died doing so. (2 Samuel 23:16, 17; 1 Chronicles 11:18, 19)
Also, it is not because the animal is dead that the blood should be poured out. It is because "the life is in the blood." (Leviticus 17:14) Clearly then, the law is about the life of the creature, not the death.
And an animal could easily lose its leg and survive, but that does not preclude pouring the blood out and cleansing it before eating the leg.