A lighter wallet.
Back when I was coming up, there was a [stillborn] attempt to change the prevailing size from 26" to 24". Granted, there are benefits with rolling over small things in your way, but for the most part, it's marketing. IF you absolutely need one and can swing it financially, then by all means- I think we'd all like to see the industry stay strong, but if you're somehow thinking a 29'er will make you faster, save your money. A fast rider is a fast rider no matter what they ride.
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Shimano : Click :: Campy : Snap :: SRAM : Bang