I can understand the use of sliders on the 3DS - the console is a clamshell and the lid requires the nub to be as flat as possible. On the WiiU though it just looks as if they ordered a whole bunch of those sliders and now have to get rid of their stock.
I'll be perfectly honest, *nothing* justifies their use on the WiiU controller, at all. They're less pleasant to use, they tire the thumbs quicker and they prevent Nintendo from putting a button under it, effectively cutting 2 buttons from the controller. It's a stupid choice, it's purely stylistic and I will complain about it as soon as I get a WiiU anyways.