Nintendo would be best off releasing a revised Wii Remote with the M+ functions built in.
Even if they ended up charging $50 per remote, it would still be less than a $60 combo with the additional benefit of making certain all new Wii owners had immediate access to M+ functionality.
It would also cost Nintendo less to have to manufacture and distribute the dongle separately (assuming it doesn't cost only a few dollars to manufacture + packaging, shipping, etc.)
The only reason for not doing this would be if Nintendo is not fully committed to making M+ the new standard for Wii controls.







