I'm a fan of Clancy stuff.
I've defended Massive's approach against the shortsighted criticism that is unable to make a gameplay abstraction from the Clancy settings, in extension completely ignoring the variety of Clancy games that exist. I will say time and time again that a Clancy setting does not preclude any type of game mechanics, let alone a loot RPG with damage scaling. You could make a Puzzle Quest type of game with a Clancy setting and it would still be okay.
But if there are zombies (or any sort of implausible adversary) in the main game then that breaks any sort of pretense of this being a Clancy game. While any sort of gameplay compromise is okay as long as it preserves the setting, this would be a direct violation of that spirit.
I personally don't care for people suggesting that type of stuff for the sake of variety or to "spice up" raids or whatever. It literally irks me to see some people salivating at the possibility. The onus is on the developer to develop interesting content while maintaining the integrity of the setting. If they can't at least do that, they fail to prove that they didn't simply use the Clancy name as a cheap flavored condom just to penetrate a market, as they are currently accused of by many detractors.