Unique IDs and where to find them

The first thing, you will hear in the HTML/CSS/JS/C Developers class room - Don't use IDs! Ever! IDs a "globals"! (Rely on classes) Why? I know 2 answers: In terms of CSS, id-selector is more specific, than class-selector. As a result - rules might go unpredictable way. So don't need Ids for styling.