It is my understanding that the Platinum upgrade applies only to availability at the time of check-in (Or at the hotel's discretion based on availability when they assign rooms.) As a platinum, I've been surprised by the number of upgrades I've received and I've not asked for one of them. I did try to request an upgrade once on a five-day Courtyard MR rewards stay in St. Louis a few years ago and was told I would have to pay the difference.

This is just pure speculation, but I think I'm upgraded more often when there is a lot of hotel traffic, and they believe it would be easier to sell a standard room than a suite to a last minute reservation. But I'm not complaining. I've been upgraded more often for a 1-2 night stay than I have been for longer stays.