I'd do it. I value MR at about 1.2 cents. Points "live" for about a year to 18 months in my account. So yes, I'd probably do $100-ish for 15k.

Something just gnaws at me when I book a room I don't want to use though. Last year, I was in this situation twice...needed 1 night to finish a Megabonus. Both times I spent a little more than necessary (low $100's) to stay at a better Marriott that we actually liked enough to spend the night there. Plat amenity, indoor pool, even managed to get a full breakfast included rate... Eliminated the feeling that I was *only* buying the points.