I have an O&G army and I never run magic. The chaotic nature of orcs combined with the need for Black Orcs to lead units makes justifying magic hard. Plus my luck is always poor and more often than not I end up with a miscast.

Fortunately I can take a bunch of dispell thanks to Morks Spirit totem so I can still put a bunch of greenskin hurt in your face. Sure I get hurt by armies with good magic but orcs are tough and cheap so it all evens out imo.