You should register, because it’s right in front of you and you won’t remember to do it later.
Even if you don’t think you’ll have Hilton stays during the promotion period, you might, and 2,000 or 4,000 points is better than not 2,000 or 4,000 points. You shouldn’t shift your choice of hotel chasing this promotion, but you should pick up the extra points for stays you’re going to make anyway.