I wrote it before.. have you seen what the normal horde catapults fire ? It's burning ammunition. So if Sylvanas had used normal horde weapons everyone in Southshore would have burned to death. And look at what the Kirin Tor are capable of with their magic or what the gnomes experimented with in Gnomereagan.
It's a harsh world but i think if you'd live there you wouldn't see plague bombs as something tremendously despicable.