We live in post-xhtml times, browsers will try to make sense and render all sorts of bad code.

>If you accidentally put the logo twice on the page, it will be clearly visible. It virtually never happens and when it does it isn't hard to track down.

You're assuming the visual difference in applied style is always clearly visible, like a repeated logo. No, it's often not, not until someone changes something in a class and the change is populated everywhere the class was reused. An ID is a guarantee it was not reused, or at the very least serves as a "do not reuse" sign.

Edit: formatting

But that's based on your own system, which may be non-obvious to other developers. The rules of valid HTML dictate the situation in which you will encounter a given ID, so this communicates something to every developer who sees IDs used in a given selector. To me that's tremendously valuable, and far more useful than trying to graft additional meanings onto parts of a single classname.

Whether that system is BEM, OOCSS, SMACSS or some other syntax, this kind of naming convention isn't a native part of CSS.