I tend to think so.
It's certainly not that Microsoft doesn't need him: he is, for the most part, Microsoft. He exlempifies the Microsoft spirit. He set the tone, the example and the standard for our work ethic.
However, he has placed the right people in postions to advance Microsoft, and his time can be better spent in other areas. Helping to save people in the world, for example.
By devoting his full attention to the Bill and Melinda Gates Foundation, he can make never-before-seen advances in philanthropy. He can also other coax other philanthropists out of the woodwork; the first evidence of this is Warren Buffet's pledging a gift of 10 million class B shares of Berkshire Hathaway Corporation to the foundation (see “A conversation with Warren Buffett” for details).