Why would I want to watch a DVD on my Wii when I can buy a top of the line DVD player for $20?
That same DVD player put into the Wii could easily add more than $20 to the production cost of the Wii and would have significantly cut down on its lifespan just as using your PS3 as a Blu-Ray player significantly cuts down on its lifespan.
The only time putting a multimedia device in a console made sense was last generation when everyone wanted a DVD player but couldn't afford one.