XL5 wrote: "unlike a conditional "line up and wait" which really is an accident waiting to happen"
I'm very curious to know why you consider this to be dangerous. IMHO it is a perfectly clear, unambiguous instruction which has been used for half a century at very busy airports in the UK without problems. I used it hundreds of thousands of times (yes, I mean that) at Heathrow Tower without any problems. What phraseology would you suggest ATC uses to achieve a situation where an aircraft may enter the runway and hold position? Remember that there is huge pressure on ATC from the airlines to achieve high movement rates so the situation where an aircraft is instructed to line up only after the one ahead has gone simply will not work.