You never know if one of the female mages would try to "experiment" with the possibility of being impregnated by a Heroic Spirit for their own greedy purposes. The odds of that happening are low, but you can never, ever underestimate how screwed magi can get.

Furthermore, that could as well be a suicidal move, since we all know that getting "replenishment" from a Servant could kill a human with ease. Even then, all is worth when it's made [b][color=ed1c24]FOR [s]SCIENCE!!![/s][/color][/b]... ooops, [b][color=ed1c24]MAGIC!!![/color][/b]

EDIT: BTW, I'm writing my post right about now, it shall be done within the next hour or so. Apologies for the delay.