Does hydrogen peroxide - urea become more active when illuminated?
I hate to answer my own question, but after some digging, I believe that the use of a light source is in fact a gimmick, at any concentration of the reagent, contradicting the only other answer so far (But I'm happy to be corrected, if I'm wrong)

This meta-study https://www.ncbi.nlm.nih.gov/pubmed/22525016 concluded that at high concentrations of the reagent (typical of in-office use), the light is useless; and there was insufficient or no data for lower concentrations (as of 2012):


  Light increases the risk of tooth sensitivity during in-office
  bleaching, and light may not improve the bleaching effect when high
  concentrations of HP (25-35%) are employed. Therefore, dentists should
  use the light-activated system with great caution or avoid its use
  altogether. Further rigorous studies are, however, needed to explore
  the advantages of this light-activated system when lower
  concentrations of HP (15-20%) are used.


A slightly more recent study came to the same conclusion https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4178330/ :


  The in-office bleaching treatment of vital teeth did not show
  improvement with the use of light activator sources for the purpose of
  accelerating the process of the bleaching gel and achieving better
  results.


I'm inclined to think that if light were useful at all, they'd see this at higher concentrations of HP too, but I haven't seen any recent studies that address the lower concentrations specifically.