It's silly how they've offered Rock Band Blitz for PS3 and Xbox 360 and then are suddenly surprised when the Wii userbase shrinks. Of course it's going to shrink if there are no new products and no innovations and you offer new products to it's competitors.
Rock Band 3 on the DS is a joke. It's got less content and gameplay than Lego Rock Band which was an earlier release. The song choice on portable consoles thus far has been pretty appauling and finally game sales are reflecting this.
Whoever is behind Rock Band is fighting a losing battle by cutting Nintendo out in this way, of course if you spend less development time on the Nintendo versions, they will not be anywhere near as popular as the PS3/360 versions. Compare this with Black Ops which has already got a confirmed Wii U version. Nintendo gamers WANT games like this, but if company don't cater to them then they can't enjoy the games they desperately want to play!