You need to read the bit above the graph again. That chart was for level 50s only, and showed no data for planetary time spent levelling. The percentage of game time spent on planets that was shown in the graph was for time spent by 50s doing dailies/gathering/wandering aimlessly, and not for anyone levelling a new char.
Sorry for the mistake - i updated it. It really makes no difference - if you visit the planets you'll see the numbers are still low. The merit of my arguement still stands - once you reach 50 = boring. More variety/possiblities in the various storylines makes the game interesting and repeatable.