People forget that in 1997 Microsoft was in the midst of being targeted for violating the Sherman Antitrust Act. The Apple deal not only helped a competitor (It's argued that Apple was a hardware company and Microsoft a Software company) but it also resolved the lawsuits at the time that Microsoft had infringed on Apples GUI.

Not only that it gave Microsoft inroads into OSX though Office and Internet Explorer. The deal had Bill Gates fingerprints all over it and it could have turned out very differently had Apple not innovated itself out of it's poor financial position.

Who knows what would have happened if Apple hadn't recovered, I think this was much less of a lifeline rather than Bill wanting his software on Mac hardware.