Instead of chiming in to the nonsense discussion above, I will try to answer the post that bumped this thread.
What happens if I close my Prestige card but leave my Premier card open - do I lose the points earned from the Prestige card?
Assuming your ThankYou points accounts are combined, you will lose the points you earned with Prestige 90 days after you close it. Soon after you close the account, you should see a notice on the ThankYou web site about the pending points expiration. Once that notice appears, all points you subsequently redeem are first pulled from the points that are going to expire before dipping into the points from cards that are still open. This is my understanding from personal experience. Call 800-THANKYO and press the option for ThankYou customer service (not Citi Cards customer service) and they should be able to clarify if this is still how it works.
If your ThankYou points accounts are not combined, I don't know if you lose them immediately or not; you can call and ask.
And of course if you make purchases and close your account before they send the statement showing you earned points for those purchases, you probably will not get those points.