"The fingernail works just fine."
Unfortunately I chew my nails.

"I don't believe that, too many DS games don't use the touchscreen at all (Mario Kart DS anyone?)."
Mario Kart DS can use the touchscreen for menu items. Minor stuff but the feature is used.
"Doesn't mean reviewers won't dock points for not using these features (even when that'd make no sense) just as they docked points for a game not being online on the GC."
Well usually when they dock points for no online it's because the PS2/Xbox version has online and thus a huge feature is missing from the Cube version in a direct comparison. Technically the Cube version is a "lesser game" and thus doesn't deserve the same score. Reviewers usually look at things from a multi-console point of view and their recommendations usually come from the approach where you can access all current consoles and thus need to know which version is the best. Mario Kart: DD didn't have a PS2/Xbox equivalent but they implemented LAN support but not online support. They did like half the work already so I can see why reviewers complained.
Similarly early DS games often got criticized for not making good use of the stylus. Often those games did use the stylus, just not in a way that was all that interesting. I think the problem there was at launch Nintendo failed to provide a game that made good use of the concept themselves so suddenly every game that used the stylus had to sell the concept even though it wasn't really that dev's responsibility. The same thing can be avoided on the Rev if Nintendo has a really killer motion controlled game at launch so that the concept is sold and thus each review isn't an analysis on the feature itself.
And I think there's a big difference between offering keyboard text input and motion control. Keyboards were widely used prior to console text input. Consoles have been using a workaround for that functionality since day one. But motion control largely is a brand new concept. Its use is not immediately so obvious. I can't think of anything where like text input people would complain "hey this should use motion control".