> It can be reusable and yet unique on each page. And those should get id's, and are not rare at all.

I _think_ what's going on here is that, in the, excuse my French, _enterprise_ sector it's very common to have some sort of an end-user configurable, enormous form (or dashboard or whatever) full of widgets that are not in any way unique. In fact in many places it's so common, that for a certain section of developers it might be hard to imagine that the opposite is actually quite common.

This is _a lot_ of "web" development. Often done by developers who, to be honest, would rather be doing this in QT (eww) or something.

Elements can often be unique today but not tomorrow as pages grow in complexity and start sharing features. Ids are fine (even necessary) for things that are unique in their very essence, like a label's for="", the target for a URL fragment, or things that need special individualized treatment in JavaScript. In the common "unique today" case, ids provide no benefit over classes as simple styling hooks and quickly become a PITA when they start colliding.