Who doesn't already have a DVD player? I think that's the main point. DVD playback requires licensing which would eat a small bit of profit annd for what gain? None that I can think of. The machine can only hold a single disc at a time and it's likely to be a game disc.
I would much rather see playback of networked media which makes a bit more sense and is something not everyone already has.
You can buy a DVD player for under $50 - tomorrow maybe it'll be $10. Good for Nintendo to not waste any time/money developing DVD playback software.
Now give me network picture browsing - it's a PITA to swap SD cards.