There are two reasons why I agree this is not a good idea, but 1 thing I'd like to point out, which is part of the idea.
-> Having to search for mobs and to try to optimize the path you take though the area in order to kill everything without having to search nor to walk 3 times the same way, is a part of the vanquish challenge.
If you have to search during 5-10mins every time, you may have to improve you vanquish skills

Having to search often happens when you vanquish for the first time, but once you perfectly know maps and mobs on it, it happens only rarely.
-> As said before, the servers don't manage mobs which arn't further than about 1.5 radar from where players are or where npcs left behind are.
-> There are still some realy annoying mobs : those who run faster than players even if they sometimes stop for 2 seconds. Usualy we try to kill them as soon as we see them but ... sometimes they are going in the wrong direction as we discover them, and it's juste impossible to follow them (and worse if there are unvanquished ennemies on the path they follow). Exemple : 1 hydra boss group in every desert area.
Since the servers don't manage them once they are too farm from players, we can't juste wait for them to come back from the other side.
Only solution is to figure out the whole path, and follow it in backward movement.
Maybe something could be done so that players can catch them (just add 2 seconds to the duration of their stops for exemple)