Interesting subject since we're constantly discussing this subject at our centre. Having 6 vertically split "upper-sectors" (FL 250+) it has a huge impact on the traffic load (in certain sectors) if a lot of A/C don't fly their filed levels.
However, me and most atco:s here try to fulfil most requests of higher levels. It then, sometimes, shoots back at us when certain centres further down the road complain that we send too many a/c on the "wrong" levels.
I personally think that, most of the time, the number of a/c flying at higher levels than filed equals the "high-filers" not capable of climbing to filed cruising level.
The biggest problem for us is that in a sector with only 2 or 3 flightlevels, and where every a/c is a potential conflict with the others, having 10 more a/c per hour than sector capacity makes a huge difference!
It should be said that a number of atco:s never climb a/c higher than filed level, no matter what. They're not many but there are a few. Guess they want to keep their back clear.
It's a subject that will continue to be discussed for sure.