They can add what they want, I suppose, but how would they market it?
A big ol' sticker on the Wii box that says "now plays DVDs"? When the PS2 could do it seven years earlier?
It's certainly not a bad addition, and I might use it if it didn't cost me any more, but it seems like it's not really something you can market easily without pointing out that there was a lack to begin with.
Also, someone should explain the Xbox 360 external HD-DVD drive to me. Maybe I'm stupid but I don't get it. How does it make use of the Xbox 360 if it's external? Couldn't I hook up a DVD player, somehow routing it through my Wii, and say my Wii is equipped with DVD playback? Huh?