Have you heard the expression "It's better to have a piece of a bigger pie, than to have a small pie"?
Nokia wouldn't be a minor player in the Android ecosystem. That's ridiculous. If they executed quickly and release some good Android phones within a year, they'd become relevant again. With their power, they could even beat Samsung, which is going to become the biggest Android phone manufacturer.
Of course they'd still need to compete well hardware wise. This decision seems to me like they need software differentiation because they know they can't compete well enough on hardware anymore.
By the time Nokia makes a few WP7 phones and sells them, Samsung will sell a lot more Android phones. I expect Samsung to become the #1 phone manufacturer within 2-3 years. This won't happen only because Samsung is growing, but because Nokia will sell a lot less Symbian units, due to competition in the mid and low end from Asian manufacturers.