You bring up a good point.
Wii would not have made a very good DVD player even if they'd enabled it, no dedicated DSP to handle AC3 / Dolby / DTS, no optical out, limited video options, and the drive was designed to be backwards compatible with the Gamecube's miniCAV discs which probably entailed certain customisations. Even homebrew which is designed to enable the drive to play DVDs doesn't quite get it working right, and I have to wonder if that is because of instruction sets that the drive was configured to understand or something - perhaps they never really planned for the possibility.
The Wii-U disc format is reported to use a variant of the China Blue High-def DVD format, so its blue-laser tech, but not necessarily blu-ray. That probably means we won't see blue ray playback, but if they got Panasonic to deliver a drive in a forward thinking way, it could have the instruction sets needed to change laser colour/attenuation, and enable reading of said discs. The U will have opitcal output via the HDMI port this time, and a DSP that could be used for processing audio and video, so it is potentially a much more capable console in this regard if they ever choose to put out an add-on or software upgrade.
I don't think they will, but it would be a nice surprise if the hardware is capable and they choose to do it someday... it doesn't personally bother me if they do or they don't. Blu-Ray players are cheap these days, and I'm betting a lot of people who care about it have PS3s. Between all the licenses that would need to be signed, its omission might end up saving us $20-40+ on a system.