You never know who might read that company card (she could be a Muslim), you don't want to break a municipal code (all holidays are created equal), you don't want to offend your cashier (he could be a Hindu), and you certainly don't want to sound like you believe in something, like you might actually celebrate one holiday in particular and expect others to celebrate it too.
"Happy Holidays" is a very acceptable phrase, no one's going to disagree with it. Images of roaring fires, laughing children, and gifts (with snowflake wrapping paper) are just fine. So to keep spirits bright and smiles merry this season, please remember that everyone is offended when you celebrate a single holiday to the exclusion of all others. Be careful, be selective, be paranoid...be ambiguous.