Only succubus, erinyse, and vampire sorceresses cast evard's that I've seen, and they only appear in certain areas. Since teh spell can be countered by other spells of equal or lower levels, plus is subject to dispel magic, the spell itself isn't an issue.
Since it's technically a summons, the ability to stack them should be considered balance wise, and the number of them that the vampire sorceresses have shoud be checked to see if it's appropriate for their level. Erinyes and succubus have three each only.