Yes, seasons greetings is a neutral term to use to be respectful to the people around you who may or may not have the same belief system as you. With the country diversifying, it is best to be mindful of your neighbor. Share the peace of the season with everyone respectfully.
Yes, it's okay to wish someone Season's Greetings instead of Merry Christmas. This would be especially true if you don't know the person at all, or if you know the person and know he or she doesn't celebrate Christmas but instead perhaps celebrates Hanukkah and New Year's Day. That being said, I don't think too many people are offended by a wish of Merry Christmas, since it's as benign as saying hello to someone, and really just means you care and are wishing them well.
In today's world it is important to be mindful of peoples views. Not all people follow or practice holiday traditions and we should attempt to be respectful to all. While many do follow these traditions we must take into account cultural and religious beliefs that vary so widely. I believe that the term season's greetings is by far more acceptable when taking this into account.
It is unreasonable to insist that everyone has to say Merry Christmas, as Christmas is a religious holiday. We, as Americans, have pushed Christmas much further than just a celebration of the birth of Christ, but that is why it is celebrated in the first place. Christmas has become a time of family getting together and exchanging gifts. Non-Christians celebrate the holiday as well. They do not celebrate it for the same reason, they celebrate the season, family, store discounts, time off work, etc. They should not be forced to say "Merry Christmas" or feel self conscious about saying "Seasons Greetings".
People should not say season's greetings instead of Merry Christmas. This is just caving into political correctness. If a person wants to say Merry Christmas then they should feel comfortable and go ahead and say it. Some people do not want anyone to say Christmas and this needs to stop.