The holiday isn't important, but your significant other is.
So for me, it's like, even if you both* hate the holiday, you don't want your loved one to be the only one sitting there without flowers or something thoughtful, so it's kinda of like, just go along with it, but never utter its name, ha!
I agree completely with your sentiment there, but am still against celebrating this commercial holiday.
Not doing so only becomes a problem, IMO of course, if it's the only occasion during the year where you'd try to do something special. Too much hinges on this day for too many people when any relationship should already be a series of thoughtful gestures and occasional surprises.