On the other hand.
Right. Maybe I'm too used to Firefox terminology, but what I'd call a "plugin" presents a new API (of one sort or another) to web content. An "extension" is solely a convenience for the user, and web content shouldn't be able to detect or rely upon it.
The former is bad, and the latter is of course wonderful --- because it's optional. Plugins have a way of either fading into oblivion or becoming practically mandatory.