In my opinion (that takes care of the disclaimer), I don't think the TL-LD500 is bright enough to run as a stand-alone taillight. I like it as a backup because under FL law a bike being ridden between sunset and sunrise requires a red taillight and red reflector. The 500, also being a CPSC certified reflector, meets that requirement.

I also like running a backup taillight because if one goes south on you, and I have had that happen, or bounces off, and I've had that happen too, you might not know. At least not right away.