Hotels will sometimes allow it, but the Diamond desk can't adjust the rate as you're suggesting. You'd possibly have more luck on the second night and checking out early, as opposed to changing the stay in advance.

But in your circumstances, where the nightly rate is the same for 1, 2, or x nights and you aren't wedded to a length of stay, it clearly makes sense to book multiple stays for additional flexibility.

It's not so much that they "want" you to do this, it's about revenue management/maximisation. It's similar to when you can get 10% off by booking a non-flexible rate, but if you'd booked the flexible rate and that fell by 20%, you'd be well ahead by rebooking.

To be honest, it's unusual to see a multi-night stay be just x times the rate for a one-night stay.