instead, they should have to guess what the plus sign means? plus doesn't globally mean "new tab". it doesn't even generally mean create. the only reason it is understood to mean new tab is because chrome has previously had a plus there. it seems to me a blank, vaguely tab-shaped button in line with the tabs is much more clear.
The current button doesn't really resemble a tab, so it could be hard to guess. Something like IE's approach seems more intuitive. Still, a blank button seems like an analogy, not a direct message. A plus sign at the end of a tab row definitely means that by clicking it, you are adding something to it. And what else could it be, if there are only tabs in that row?