I think it would be safe to say that what a lot of us object to is not "Happy Holidays" or "Merry Christmas" -- it's not allowing people to say "Merry Christmas", or people being offended by what is essentially a cultural wish of goodwill toward another.
Tell me Happy Holidays. Tell me Merry Christmas. Tell my Happy Haunaka, Kwanza, Solstice, or keep your mouth shut and don't say anything. Just let people, who mean the OPPOSITE of harm, wish you well, and who cares what the reason is?