I have several pagan friends who don't like this day, and I understand why they don't like it, but I celebrate it not because it's a saint's feast day but because it's essentially Hooray Irish Culture! day. It helps that I'm half-Irish myself.
If someone wants to be honorary Irish today day, I don't mind. Being upset that someone wants to temporarily join my tribe is like being upset at girls who suddenly like comic book movies because Robert Downey, Jr. and Chris Evans are hunky: look, I don't care how you got here, just pull up a seat and join us.
PS: I still think that this blogpost is my best St. Patrick's Day post, ever.