Thanks for the responses. Most of my holiday trips are kind of spur of the moment and I find myself looking for things very last minute. I think the fact that most of my recent travel has been to eastern Europe in the winter may account for this . The rates are double in the summer. I wouldn't put Madrid in this category though. As a result, all of my Marriott stays in Europe this year have been <100 USD per night so I would rather earn the nights than spend 20,000+ points.
It does seem much better value to use the points at higher end properties (London, Paris, New York, Cosmopolitan, etc) but I find myself staying at these places using other methods. For instance, comped rooms at Vegas or AirBnB in Paris.
I guess this is the reason I tend to horde so many Marriott points!