@Chris: I think a legacy branch would be intended for users who desire to run stable software -- you could infer that such individuals would be *more likely* to upgrade because of that vested interest.

@Dougal The longer period of time a legacy branch is supported, the better -- for both the individual/business running WP, and the team maintaining that branch. A 3-4 month window is too short for companies that use WordPress publicly. Personal bloggers have a much-easier time upgrading their installations than businesses. Time spent trying to always be on the most-current version = time *not* spent doing other things, like running a business.

@Randy You make excellent points about how feature-rich WordPress has become, especially when describing 2.6.x. By the time of that branch, WP had gained enough useful, regularly-usable features that there wasn't a huge push to upgrade. This is what kept me on WordPress 2.6.x for the longest time (that, and my breakable custom theme). I don't know about your idea that more people would adopt WordPress due to the existence of a legacy branch. And if this did increase adoption, it would surely be just businesses who are currently weary of running the latest version of any open-source software.

I think in general, I support this idea, but I also know I would never use it (my own blog ventures are not highly impacted by upgrades).