I think the issue is more that the battery runs out and now it's 2007 again and you start overwriting img_20070101_01.jpg ; last-directory-entry++ is a bit more robust.
>>bux93+(OP)
One upside is that it hopefully prevented developer to ship half-baked software that rely on filename and can't handle duplicate name gracefully.
You can't prevent collisions (multiples sources/counter reset/date reset, etc). So it's actually nice to have an unforgiving standard that will bite you if you make unfounded assumptions.