I will characterize his playing with appendicitis as an unwise thing.

So, here's a random theory on why he's trying to play with it: he wants to play and try to win the WTFs so he can beat Federer to "winning everything."

The only notable title that Nadal is missing is the WTF title.  In contrast, the only notable title Federer is missing is the Davis Cup (no, I don't consider the Olympic Gold medal as being notable; it's just too infrequent to consider).  The chance of Federer winning Davis Cup is very high this year (though with Wawrinka stinking the place up recently, it may be more difficult than it should be).

Nadal made the finals last year, and he probably thinks he has a good chance against a distracted Papa Djokovic.  So, if he can win there, he will win all notable titles before Federer has a chance.

Either that, or "appendicitis" in Spanish means something very different than what it means in English.