The problem with this is that the period where the guy you just killed is a ghost, is important. Especially for classes that don't need a lot of down time. Healers, who just need to healing sleep, and are immediately ready to go. Or conjurers, who can't conjure as ghosts. This is especially imporant in raid situations where with 4 people defending, you may be able to do so indefinitely because they just keep coming back after forcing an unghost in an explore area.

In general, the ghost lag time prevents mothing of various sorts* and that's pretty important.

* Mothing, for those who don't know, is attacking something over and over, dieing each time, but doing a little bit of damage in the process, so that eventually the victim dies.