Leaving the disc in the Wii U is absolutely harmless. Heck, it even covers the lens from the disc drive away from the small particles from settling on.
I've never heard about this before but it seems like one of those silly made-up rumors, like the one that "using the 3DS' 3D will give you eye cancer." Hint hint, you can't get any type of eye cancer just by looking at something.
Back to the point, I've had my Wii U almost since it was released and there's almost always a disc in it. It has absolutely no problems reading discs.
That would be a huge design flaw if that really happened since so many people do it. I leave my discs in my Wii U all the time and have never had a problem with them being read.
The only time you shouldn't leave a disc in the console (any console) is when you want to move the device. It'll definitely mess up your disc, with the added danger of the lens being ****ed up.
Leaving the disc inside a machine that won't be moved won't do any harm.
The only time I would ever worry about a disc being left in a console is when the console is being moved. My dad once removed the WiiU from its previous set-up without warning either me or my brother, and his WiiU acted a little strangely with Mario Kart 8 after that, for some time. It seems to function well now, though. No more troubles.