I'll venture a guess and say that the sliders are probably low quality therefore not allowing fine movements of +/- 1% in their travel. That would make editing certain features extremely difficult with the sliders alone. Hence the addition of increment and decrement buttons as well as sliders.

High quality pots that offer fine movement control cost lots of money and when profit is a consideration more than build quality, manufacturers make sacrifices. Does Yamaha make more profit or do they make less profit and use higher quality buttons, switches, and materials? Given Yamaha's propensity for using cheap parts I'm sure they opted to go with the cheaper components. Sad, very sad.

When you consider the price point the Tyros 3 falls within there is no reason it shouldn't be built using a high quality lightweight metal frame, 1% pots and switches, and a great feeling keybed. Consumers are being soaked and Yamaha is laughing all the way to the bank!