Thats what people said about the Zune and the XBox. True with the latter (no one can say the 360 isn't a huge success) while very much not true for the former. Despite pushing the Zune for years it never took off and was recently abandoned. What makes you think Windows Phone will be more like XBox than Zune?
Simply that there is way more at stake, and Microsoft won't give up as easily.
Zune was a media player, the market for which is being destroyed by smartphones (why carry around two devices?). So merging Zune into Phone makes sense- I just wish they'd stuck with the Zune name, in all honesty. But there's probably a corporate angle to why they didn't.
Importance. Microsoft (rightly) considers smartphones to be a serious, if not existential, threat to them, breaking their near hegemony on personal computers. Given the importance, Microsoft should be willing to throw vast quantities of resources at WP7, to save themselves.
In fact, the same arguement as been made regarding G+: Google sees Facebook as an existential threat, so they'll throw money, time and engineers at it until it works (or falls so flat that there is no chance whatsoever).
