Actually, this really doesn't affect me. My entire family already have 4 DVD players (and 1 VHS player) and my PS3 can already play Blu-Ray.
It doesn't affect you, but this isn't about you or me. This is about options and Nintendo providing them where possible.
If the hardware is technically capable, and I'm willing to pay extra for that functionality, it makes sense to have such functionality... especially if I'm willing to pay extra for it, then why not allow it?
Wii was capable of playing DVD's. And while it didn't really matter to alot of people whether or not you could actually play a DVD in it (who didn't have atleast 2-4 DVD players already in their home back in 2006?), I imagine it was still a rather popular homebrew app that allowed for free DVD playback.
The main difference between now and then with DVD playback, is that not everyone has a Bluray player for every TV in the house. You can't go to the store and buy a standalone bluray player for $20-30 and put it in the kids room, kitchen, spare room or the garage. Capable Bluray players require internet connection and decent processors for apps, extra features and things of that sort.
WiiU will possibly be capable of playing back Bluray movies and being a more than decent Bluray player. A bluray player that can stream a Bluray movie to my (and possibly 3 other) controllers all at once. It could go one step further and put the Bluray extras, such as Live quiz and stuff onto the controllers and have a 4 player quiz while the movie plays full screen on the TV.
So if the end consumer is willing to foot the bill for DVD and/or Bluray playback on an individual basis, then why stop them? why not give them the option and make a little extra profit off of it in the mean time?
edit:
Adrock... get outta my head.