[Tashim]

Actually, you can use the simulacrum repeatedly, but only if it can gain agro in one round.

It is based on the hatelist of the target. In order for the simulacrum to exist, it must be on the top of the mobs hate list. The reason you see it vanish when you cast it again, is because the mob hates you (or someone else) quite abit.

You can verify this by casting the simulacrum on a mob, and just standing there, not doing anything at all. Recast the illusion, and the mob may briefly look at you, but it'll turn to face the simulacrum again.

The Simulacrum can be used in several ways, but it does require an understanding of agro, and the ability to manipulate it with finese.