Please remember there are large numbers of users who cannot upgrade the browser they use at work, and there are valid reasons why their IT departments do not let them upgrade.
If your webapp is for entertainment, social, or otherwise not useful for corporate, government, etc. users then you're probably right. But otherwise you may be punishing users who have no choice in the matter, hate IE6 as much as you do, and use Chrome at home.
I agree about code bloat, etc., and if there's a clean way to maintain a separate branch or plugin to support IE6/7 users that's great, but if a library doesn't support them, I simply can't use it given my userbase.