I think what gexer64 meant was that, let's say someone summoned a servant who impersonated the Goddess of Mercy, when she returns to the grail, it will be purified, since she is suppose to be the avatar of goodness.
However, when is the last time you heard of someone who is the embodiment of virtue that is not flawed? Most good heroes became heroes in the process of atoning for sins. Or, the hero was noble, but was corrupted and thus fallen. No hero was truly flawless and pure good. Only gods are, but then again, the gods like the Greek gods have their share of follies. EVEN if you find someone that is pure goodness, will the individual make a good servant? To be good is to be pacific and merciful, like Jesus in the bible. Now you see why the Einzberns opt to summon something pure evil?