I take issue with this column. By using one holiday greeting, which is not religiously neutral, you ignore the people that are of a different faith than which said greeting applies to. If someone were to greet me with MC, instead of Happy Holidays, I'd get on the defensive immediately. I'm Jewish so I automatically correct them. It's the right thing to do. Use Happy Holidays. That way, you are sure to not offend someone that isn't of the same faith that MC is applied to.
The worst is when one is wearing their kippah and someone still uses the non-appropriate greeting! Believe me, that happens more than I'd like.