No polarity protection is one of the short comings of many LED lights. In an emergency or if one simply gets distracted, it is too easy to put the cell(s) in backward. I've never done it, but I hate having to worry about it.
If the light is one that has the + terminal at the tail of the light, I think it is critical to have the protection if reverse-polarity will damage the light, since this is non-standard.
If the batteries are inserted in the conventional manner then, I think it is still very important to offer some protection.
Unfortuately I recognize there is not a great way to provide this protection given the wide variation in cells people use and the small size of the driver circuits, the effciency people want, and the amount of power modern cells are packing.
The easiest thing is to have manufacturers only design lights such that the cells are always inserted with the positive terminal to the head.
The physical protection that requires the battery to have a positive nipple is also good, let the battery manufactures make their cells accordingly.
Let hope the manufactures take the lead and realize it is to their benefit to make their lights reliable and safe.