Yeah, I wonder about this myself. It's obvious why a player's character should remain in-game for a while after they disconnect, to prevent abuses of this sort:

Raider 1: Okay, guys, boss is about to do his ABSOLUTE ANNIHILATION attack! The boss is at 20 annihilation stacks because none of us can dodge for crap, and our gear is all horrible. There's no way any of is is going to survive, so it's time to bring out the industrial strength cheese! Raider 2, pop Halowed Ground. Everyone else, get ready to yank your ethernet cords! 3... 2... 1... PULL!
*Everyone disconnects; their characters instantly disappear. Players log back in five seconds later. Raider 2 is standing in a cooling pile of molten rock, filing his fingernails.*
Raider 1: All right, the attack is over! The fight is a breeze from here. Good job, all!

However, I see no reason why a player who disconnects can't rejoin as soon as they have the opportunity to do so, so long as their character remains in game to soak up any punishment. Why is it necessary to wait for the zombie character to finish its disconnection period? Player should be able to get right back into the game. So long as the character remains in-game, all opportunity for abuse has been covered.