I think this has more to do with the massive cost of the screen and not wanting consumers to see a game requiring a second one. The Wii Remote / Nunchuk is already excessively priced, the tablet would probably cost $100 stand alone (6" touchscreen is not cheap) and that will turn way too many people off.
Myself, I like this being separate (as long as Wii Remote remains an option). It's not super heavy but I can see a problem with marathon Zelda sessions. It also mean my investment in Wii Mtion+ Remotes isn't lost (not an issue for all people). I think it's actually kind of cool that we still get to use our old controllers from the previous system with the new one, even PS2 didn't entirely do that (they added the pressure sensor buttons). It also makes it easier to see both platforms supported (one set of peripherals in Toys R' Us).
Where I would call this move a mistake though, is things like Madden. Selecting plays separately and such (what was pushed for Sega's VMU) or other small scale uses of the screen are out. Though as I said above, the biggest issue is the price of the screens I imagine. The bandwidth for the stream probably also factors into it. Remember, in Japan's small apartments (or dense urban Europe / America) radio interference is an even bigger deal, and adjacent apartments all throwing out multiple streams could start causing some real issues.