Gilbert skipped the tour and rode the vuelta entirely for worlds preparation.
Spartacus was in form at the tour. Maybe not spring form, but he was still good. Compare his tour ride to his vuelta ride.
Nibali was going to ride the tour originally, but liquigas wanted to be well repersented in italy (and have further support for basso) so called nibali up last minute to replace pellizotti.
Turned out to be great for nibali.
Weather you want to accept it or not, most cyclists, the younger riders included - prioritize the tour de france over all else. And though I agree the giro is at least equal in greatness, the Tour de france will always be considered the crème de la crème in GTs. It's easy for us to sit here on a forum and claim riders would easily win the giro and vuelta if they skip the tour.. blah blah blah, but in reality, this doesn't happen. People expecting to see gesink, nibali, anton etc to go to the giro and win next year, rather then rider another top ten at the tour better be prepared for disappointment.
Sure nibali got a 1st and 3rd at 2 GTs this year.. better than any other cyclist right..? but I wonder how many people here think he is the number 1 GT rider in the world
the youngest's logic is simple - the best riders all ride and attempt to peak for the tour, because the winner is considered the best GT rider in the world and it is, the most prestigious race.
I'm not even a tour fanboy, i prefer the giro and even the vuelta some years. But I understand why the riders, and most fans hold it in such high regard.
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BTW I agree gesink has to improve in his TT. But he is definitely a serious GT contender, and imo will compete with the best at the tour in the coming years. I hope to see him compete with the best at all 3 GTs throughout his career though.

And I refuse to believe Andy is a far and away better tt'r than gesink.