Well that actually is my 'argument'. The logic being that when M+ first came out if they had released the modular attachment and the new Wiimote Plus at the same time, existing Wiimotes could have been upgraded and all new owners would have had the new Wiimotes.
Given that the Wii install base was significantly less than 75million when M+ was released, a higher percentage of the customer base would have had M+ capabilities if this had been done (and there's no valid reason why both couldn't have been released). Therefore potentially making M+ games more common/the norm.
I'm arguing this may have been a missed point for Nintendo. Whether it was or not is now arguably immaterial, but I think they are a bit late in releasing this now.