Depending on Berdych or Del Potro - Nadal could fall out of the top 5. That's what a QF at Australia was worth. It's a harder road for him back up if he falls out of the top 4, since he could play Djoko in the QFs.
He was going to fall out of the top 4 regardless really. Ferrer was very close -- getting to the Final again and defending the points in his first tournament back was a long shot really. Whether he lost no.4 at the AO or at IW or Miami etc. the likelihood of him being able to avoid it was low. As for avoiding Djoko in the QFs - he'll play 1 or 2 HC tournaments (IW and/or Miami) and after that it'll all be clay. Think Novak might not be as happy to see him as a QF opponent either. Fact is on clay not being top 4 is the not the biggest thing for him.
Ranking alone is not a good enough reason to risk his body in the heat on those hard courts over 5 sets. He has nothing to defend after RG practically so makes more sense to focus on clay so he can defend a lot of those points (and win more in South America) and then gain points for rest of year after RG.