6 Lings won't require loads of SCV's on a Bunker and If you have 400 Medics healing 1 Marine, the Lings will go for the Marine instead of the Medics.. I really wish Blizzard would make a patch for that, but I doubht they will now that all the other games are coming out.
And as for the Ling and Bunk arguement, pan just pointed out 1 of the things most people overlook, the travel time and most of the time if your playing on a big enough map, you wouldn't be able to scout the enemy before you got 6 Lings.