2004 for Federer is 2010 for Djokovic and 2009 for Nadal : have those guys been examplary in Davis cup since that age ? I think they have not been examplary at all, sorry to tell you but actually in last two years Fed has played Davis cup more than them.
Actually, yes: Djokovic played all the ties in 2011, until he got injured during a Davis Cup match against Delpo, which cost him a couple of months at the end of his fantastic season. He played that match only a couple of days after winning a grand slam trophy in the US... I call that exemplary.
He decided not to play Davis Cup in 2012 because of the Olympics. But in 2013 he's back playing for his country.
Djokovic won the 2013 Australian Open on a Sunday, and, instead of relaxing, or celebrating, or savouring his triumph, the next day, Monday, he flew to Belgium and on Friday he played a Davis Cup match.... Many players were impressed, thinking he would have pulled out after winning the AO. But he didn't.
So, yes, Novak has been commited to Davis Cup for years and he hasn't stopped now that he's the #1 player in the world.
I think Roger has every right to manage his schedule as he sees fit, I'm just mentioning that Novak's commitment to Davis Cup, now that he's at the top of the ATP rankings, is higher than Roger when HE was at the top of the rankings, in 2004. Roger was very commited before, as you say, but as soon as he made the #1, his interest deminished. Nothing wrong with that, of course. Just don't pretend that Novak's interest in Davis Cup took the same dive, because it didn't.