The fact that they can still hoover up your browsing data the same way as in normal mode.

Google Chrome has defaulted to disabling extensions in incognito mode as long as I can remember:

> Allow in incognito [ ]

> Warning: Google Chrome cannot prevent extensions from recording your browsing history. To disable this extension in incognito mode, unselect this option.

It probably has something to do with (some) extensions not being very privacy friendly. Some extensions spy on the user in return for the utility the extension provides (remember the Stylish addon fiasco recently?). I'm not certain of the reasoning behind the decision. Also: it's possible to enumerate/detect what addons are installed in a browser with javascript (and thus fingerprint the user), although I think that bug was patched in Firefox and Chrome.

What addons are installed is one criteria of bowser fingerprinting.