This can be used to steal someone's browser data. Today I just allowed my friend to use my laptop just to check mail (he was in hurry), he opened chrome and by mistake had signed into sync available at the top right corner in dev version (20+). From that moment all my browser data has been uploaded to his gmail account. I found it only later but by the time everything has been upload to his dashboard. Luckily he was my friend so I asked him to delete my data in his dashboard. This is a serious flaw. You must be able to disable it if you don't want the feature. Google why are you so stupid.