Such a pair should be considered one button (just like the dpad is one "button") since you only have to remember one to know both.
You still need to know what they do and where there are, just as any other button. You can't separate function and position. There's still a degree of learning involved in both a TV remote and a game controller.
Casual games on the Wii are designed around motion controls, rather than button presses. However, if a casual gamer wanted (and that's the key here) to move on to traditional games, "graduate" from casual to "hardcore" games if you will, they can and buttons aren't going to stop them. Why? Because they can figure out a regular remote (position and function of certain buttons). Because every seasoned gamer had to once. Were we deterred by the SNES? Still, that's
if they want to and I'd say, most of them do not. They'll keep playing Wii Sports.
Also consider how people remember the controls. They will often forget details, remembering only "press the button near the thumb" which means they are screwed if they have to remember one button out of a button diamond, they will remember it's in the diamond but not which button in there. On the GC you could remember "big green button", on the Wiimote you can remember "big thumb button" and have more or less a unique grip for each button...
I've already adressed this: There are ways to make it easier for people if in fact, people need it to be. Convex/concave buttons (like the SNES), different shaped/sized buttons (like Gamecube, but not as ugly) and so on.
There doesn't need to be a diamond arrangement (though I'd like to point out again that the DS has one). Take the Wii remote and add 3 "satellites" buttons. If, at that point, people still can't figure out that they must press the big, round A button when an onscreen indicator tells them to press the big, round A button (i.e. Wii Sports), especially when their thumbs rest directly on the big, round A button, and the other satellite buttons located around the big, round A button are programmed to do nothing, then that's their own fault because they are stupid and they have no business being anywhere near a videogame controller. I would wager that very few people fit into that catagory and I don't think a whole controller should be designed around accomodating/coddling them.