Final update:
OK, I get it now - that about filling in the "Start in.." box is needed in order for the task to run, but the repeating ad infinitum thing is different - figured it now:
Consider this task trigger:
Start at 10:00 and repeat every 2 minutes for 10 minutes.
If you refresh Task Scheduler at, say, 10:05 you will see the "Next run time" as 10:06, as expected. If you refresh again at 10:11 you will see it is blank. The task has finished.
Now set it to repeat on a daily basis, and refresh as 10:11. Instead of seeing the "Next run time" set to 10:00 the following day, in fact it is set to 10:12, and continues to re-set itself every 2 minutes BUT the task does not actually run until 10:00 the next day!!
Daft, or what?