Microsoft have such a bad reputation for screwing their business partners so many people are very negative about the Nokia Microsoft partnership.
But I guess Nokia is in a hardware business and makes its money by selling phones. Microsoft needs Nokia to be successful in order for WP8 to be successful. Nokia is in a better position against Microsoft compared to HP, Dell, Lenovo because of brand and distribution channel (in Europe, Africa, India, China), wireless IP (as in LTE compatibility), and maps (as seen in Apple maps fiasco).
Windows Phone 8 and Windows 8 is out and in the past month Nokia is in attack mode. Doomsayers of Nokia mostly already shut up.
Many consumers will be introduced to W8 via Nokia, but I don't believe it is useful or accurate to say one "needs" the other more - it is a strategic partnership that bodes well for both, and one where both bring respective critical capabilities to the table. My prediction - both will increase, in rough correlation, over at least the next two years (or so). Nokia will increase more.
That is just too big of a pill for MS to swallow. They would have virtually no cash left if they did that and with the way markets or shifting they know they need the cash for the future. The partnership is the right move for them.