Although an important distinction, I think we might confuse some readers by mixing apples and oranges. Now we're talking about differences in flashlights, rather than differences in battery types.

That said, back to batteries, you can also be left in the dark with an unregulated light if you're using Li-Co rechargeable RCR123's if you run the cell down enough to trip the low voltage protection circuit built into most RCR123 cells. At that point, switching modes won't help. The only way to reset the protection circuit in the battery is to recharge the cell.

This wouldn't happen with an IMR-type rechargeable cell because it doesn't have a protection circuit. Nor would it happen with a non-rechargeable CR123 cell. (Not being able to turn the light back on, that is. The light itself might still switch off because it has its own low voltage cutoff circuit, but in that case, switching to a lower mode would probably let you continue at a lower illumination level, like Haertig was describing above).