also, when you think using IDs, you think "this component needs to be red with a blue border". It's impossible to think "reuse". When you think using classes, you can think "this component is an important button" which makes 2 clases "important" => "red" and "button" => "blue border". Of course sometimes a component is unique in an app, but if you get the habit of using IDs you _can't_ make it reusable.

Hope this helps !