Problem with buying a dedicated bike light is that you tend to pay alot for something with great output. There are some great designs out there but the problem is that LED torches are simply a better output/$ ratio especially in the sub $100 arena. I mean my dual P7 setup can hold it's own against anything other than top-tier lights like the secca 700 or other $300 plus lights, but my setup cost half that. It gets even more efficient to build your own LED based lights.

The bike light will have the advantage in terms of mountability and UI for biking but for me its not worth. And the big reason they are more expensive is because of all the R&D that goes into them and because it's difficult to stay on top of LED techology in the market.