If you're a power user, this kind of "can't leave me" sites are a rare and minor annoyance - you know that they're sandboxed and can't do anything bad unless you allow it, and you'll have it cleaned up WAY faster than the time you'd have spent managing JS exceptions and debugging sites that broke in non-obvious ways.

And if you're setting a computer up for a non-poweruser, you can't deploy uMatrix because you really can't expect the user to do said debugging.