Since depletion can be 'repaired' by sorcerers in their next movement phase, by the time the next combat phase comes around the defending units are back to full strength and can repulse the attackers. Fortresses especially are virtually immune to attack. Some units, however, are relatively easy to destroy, to wit, lone magical units. In fact, the whole system seems engineered to prevent magical units from exercising any independent action. They usually are not worth the trouble.
Cloning allows multicolour sorcerers to split down into two or three one colour sorcerers.
It is hard to think of situations where this is worthwhile, even though it does enable you to convert more white hexes to colour. In theory, it enables more units to be conjured, but this is the least of your worries. Many games, in their later stages, see 'expendable' magical units fading away through attrition. The stacking and combat rules forbid more than two units to attack a given hex, so surplus units are quite useless. He exists merely to stir and unfortunately requires an extra player.