Assuming that you don't want the seatpost you already have, that you don't want to buy a new seat post just to get a tail light, or at least that you don't want to drill holes in the seat post you have that you count on to keep you from catastrophic failure while riding. Or, that you don't want the light that goes in that LightSkin seat post, or that you might prefer to upgrade later even after this product fails in the marketplace.

But, if you're buying a new bike, aren't particular about your tail light, and will not use a saddlebag or rear rack, it looks like a useful option.

[edit: and also apparently, assuming that you want to remove and replace your seat post when you need to change the batteries.]