This spell was very useful for blocking strong summons (such as Fire Elementals) that could push parcels, as they would not push away the skeletons. Although the skeletons used to die quickly, they could be used to block monsters, as the monster would usually retarget to another one and waste time getting to it. It could also be used to block paths and since there was no limit of the amount of Skeletons you could create it was a spell that was a lot of fun.
It was removed from the game with the Winter Update 2010. But a curious fact is, that saying the magic words "exana mas mort" actually tells the player the spell exists, but you must learn the spell first (which is no longer possible).