Ghazis are good for wood ambushes in defensive battles. You have to position your main force next to a wood and hide the Ghazis there. If you keep your general at that side and leave your flank seemingly exposed (apart from the forest) the AI may start maneouvering up that flank, if you're lucky... straight into the waiting axes of the Ghazis. Basically in the confusion of the wood, and with the element of surprise they can be quite devastating, especially if it was cavalry that blundered in there.