I would strongly recommend that anyone issuing or receiving conditional line-up clearances read
the accident report for G-SSWN, the Shorts 330 involved in a fatal accident at Paris de Gaulle.
It does not necessarily make a case for avoiding such clearances, but it does show how spectacularly things can go wrong if someone loses the plot.
The recommendations are worth considering for anyone involved in designing procedures for conditional line up, in particular that:
* the instruction include the holding point that the aircraft should cross to line up
* the instruction to line up behind another aircraft uniquely identify the preceding aircraft.
In other words "line up number two" is asking for trouble.