The biggest thing I have against single-sign-on is that I need different levels of security for different sites, and I want to keep the sites compartmentalised from each other.
For instance, I want high security for my email account and access it only from computers/devices that I have control over.
However, I have private playlists on Youtube that I may want to show to a friend, on a third guy's (two degrees of separation) computer. I don't want to have to be afraid of logging into Youtube on that machine because that computer would also get access to my email.
When I am on my trusted home computer, having different accounts for different things can get cumbersome with those sites that force single-sign-on on you!
Yes, while I could use the Incognito mode in Chromium to separate my logins -- it does only separate [i]two[/i] sites, and I would have to login each time I need a new window in incognito mode.
It would be much more convenient if I could have different "realms" or "personas", where I could browse each site in its own realm.