Its fascinating to see the commonplace "own a Wii and a HD" concept repeat itself over and over again. I don't understand why so many threads insist that the Wii and the HDs are competing, in the traditional sense.
The relative lack of genre overlap would, of course, eventually minimize the weaker (Wii) consoles ability to compete (from the 3rd party publishing perspective) in certain genres, as well, since a superior (technologically) platform exists in the same household, amongst so many mature gamers.
I recall a report issued by NPD which showed that, even in 2007 (I think early 2007 is when the report was), something like 3% of all the reports respondents (the respondents were the general populace, as I recall -- not all of them even played games) owned "multiple next gen consoles" (7th gen), out of the 19% of total respondents that even owned consoles, let alone 7th-gen consoles. 3/19 is over 15% of 7th-gen households owning more than one 7th-gen console, assuming that all console households in that report owned a 7th-gen console (which I doubt).
I bet that 2nd console is almost always a Wii... although I dunno if the % would have gone up or down by now... (I would guess down -- i.e. more Wii-only owners as a %, but it could go either way.. especially by the end of the generation)
