There are benefits to getting your own back. Especially if it is blood.
The bigger the operation, the more blood gets spilled. In procedures like open heart surgery and major trauma, blood loss can be so great that large quantities need to be replaced.
Blood transfusions are often the preferred option. But in a minority of cases there can be adverse reactions.
And then there is the cost. As Professor Terry Gourlay puts it:
Blood is not cheap — via redwolf.newsvine.com