Maybe by the end, its all bots because people have gotten lazy throughout the season... and them lazy folks don't mind infesting their comps with spyware, and the like. I'd say many of the top people got there legitimately. I know when I was leveling my zon there was a very high druid and zon pair that played (non-botting) - it has been awhile since I have really tried to level though.
I think you have missed the point or something... What do you mean is it worth trying legit? If you want to hit lvl 99, try it. If you want to beat Hammerdin Bot 56 to lvl 99 and only put in 3 hours a week, don't hold your breath. If your interest lies in leveling do it, and don't worry about xxHammerdinxxNoob and his bot... Who cares about the bot competition... honestly? If you hit level 96, 97, 98, 99 it will be awhile before the botters (if that) kick you off the ladder.
You don't need to play 24/7 - many people do have lives, however yes it is a time investment. If you don't want to then don't bother, but don't worry about the other people on battle net and how they achieve their ladder status...