Hilton does this too. I'm Lifetime Diamond and before every reservation if there are upgrades available, they email an offer to upgrade. I always use that as a barometer on how much upgrade availability is there. In those cases, I tend to get the upgrade since I'm not inclined to pay to upgrade and a bed is a bed so if I'm not upgraded no big deal.

Conversely, flying is different. If it is a long flight and it looks bad for upgrading then I'd probably fork out the extra money.