The point is though, that for a lot of players (Although not all) the temptation to skip the zombie part, and take the option to try again in a new city in hope of getting perks as a surviver, is too great. Therefor there will be a lot less player zombies then you might like, and let's face it, from what you just said player zombies have a lot of potential for fun situations.
Therefor, if there were a some perks you could only get as a zombie, people will still play as a zombie, while better perks shoud be open to survivers to encorage people to survive.
Well, there is the inherent perk that it's officially sanctioned griefing, and lots of people like to take on monster roles in a game when given the option anyway. I would be surprised not many people went for it, especially if it the game asks you to pay per character rather than subscription. I bet a large portion of people who die early in the round would opt to play as zombie, which is kind of neat since the older monsters would be the deadliest as it always should be.
I can't actually think of any reason having zombie survivors earn perks and graduate from city to city would be a bad idea, but it makes me uneasy somehow. Although it would be neat if zombie veterans could be summoned by GMs to be placed in a fresh city.
One thing I think would be really special about this game, is it would enrich itself over time unlike most MMOs which seem to stagnate because people reach level limits/crunch the math on best character builds/beat all the instanced dungeons/etc.