You can't conclude that. Nadal played Halle, he didn't play the Olympics or US Open. So is Halle more important to him?
DC comes after WTF. His physical status could have improved. And one match will be a lesser strain than three.
Halle wasn't more important for Rafa than OG, it was just better preparation for Wimby than what OG would've been for USO.
for Nadal and Djokovic only, their attitude is too often generalized now, other players really do care a lot and try to play their best in the WTF and being there is like the goal of the year (Tsonga said that he will play more tournaments to be sure to be there for instance, and you can't say that it's not important for Murray as it's the second most important tournament in his own country).
When you talk of scheduling and slams, just look how loaded is the period before Roland-Garros and Wimbledon, top-players don't play more in the current period, quite the opposite.
Besides, as for Nadal and Djokovic, I think the way they dealt with the WTF last year had something to do with their long-term plans for the Tour : to shorten a lot the end of the year they had all interests to make people say "this period is useless : top-players are just too tired" ... and they clearly managed very very well to make people say that as their attitude, and especially for only one year, has been actually very well generalized.
And now Nadal and Djokovic's plans to make the WTF be played just a few weeks after the US Open are in very good way from everything we heard.
Rafa and Nole belong to the best four players on tour, if they don't play their best tennis at WTF, WTF loses its credibility as a semi-major. Yeah, it's nice to see other players in semis and final, yet e.g. last year WTF as well as Paris and Shanghai M1000s seemed to have more fluke semifinalists than e.g. Indian Wells and Miami M1000s.
And I doubt Rafa and Nole tanked last year at WTF just to make ATP shorten the season.
Only thing which is weird is how dismissive many tennis fans - and even supposedly better knowing "experts" - are dismissive of tennis tournaments which aren't the biggest. I sure as hell have never heard a soccer fan whine how national leagues are meaningless and superfluous since World Cup is so huge.
Bad comparision, footballers play their best football in national leagues as well as in WC. But tennis players don't always play their best tennis in WTF. I'd be OK with WTF if players played their best tennis there.