Somebody mentioned earlier - though I haven't the willpower to trawl back and find it - where does the "AND WAIT" bit come into it?
Why does the phrase "Line up" (whether conditional or not) need any extra comment - it's NOT a take-off/departure clearance - shirley, it just means enter the runway, point in the right direction, sit still and wait for a further clearance.
Has there been an incident in which somebody didn't wait, or didn't understand, after receiving just a "line up" clearance?
(Yes, it was deliberate).