As you said.  The undead outnumber our forces three to one.  And there are a<em>lot</em>of them.  They can't bring all that to bear at once, plus they'll have clerics with them and Protection from Evil up through the Hallow effect.  When we flip the switch, we're pretty much guaranteed to wipe out the vast majority of the undead; our forces won't even need to go up against a third of an inferior force while at a major advantage.