Tired,
Let me rephrase YOUR question, why NOT give 5 or 6 aircraft conditional line up clearances? If the runway is only being used for departures, and there is no ambiguity and no chance of misunderstandings, the procedure is safe and efficient. Workload is reduced if you can give a series of line up instructions sequentially.
Air Departures is all about priorities - using the available RTF time effectively. You need to time your instructions carefully so that no time is unnecessarily wasted. You cannot afford to be issuing marshalling instructions when you should be issuing take off clearances. Those lost seconds mount up over the course of a day and become significant, and lost time as we all know is money and lost capacity. If a pilot knows where he is in the sequence, then he can plan to have his checks complete, line up on the runway promptly, and roll immediately on receipt of take off clearance. We get minimum departure spacing, you get minimum delay.