Well, Nadal is not saying that the slams are less prestigious, nor that slams are less important for the legacy of the player. I believe he's just saying that winning the Olympics is more important to him than winning a slam. Nadal may actually believe this. I know a player like Massu or maybe even Gonzalez would say that the Olympics are more important to them, and not just because they have had success there. On the contrary, they have been successful there because winning a medal for their country is so important to them and they found great motivation and form for the event.
The only thing that is odd about Nadal's statement is how much more irregular he was about playing Davis Cup, yet this is still a tournament representing your country. But Nadal is a pretty odd character who probably has like 10-15 "most important tournaments," depending on what city he's in and what cliché press conference he wants to give.
Are you talking about a man who alredy won 4 DC and a olympic gold medal (in a hard court)? How many players today have done so much for their country? Criticizing is free, but if is about Rafa it's a inexcusable obligation, I suppose.