Why even going through all this trouble when there is a bunch of inexpensive designated bike lights? Looks like an additional mount isn't a concern here. If it were - there are bike lights with strap mounts, really quick and easy to install and remove the entire unit (e.g. Niterider Swift series).
Remember, just because you can make a mount out of a soft drink bottle, doesn't mean you should (that was fun to watch though).

To answer the original question:

Pros: you'll still be probably seen, potential savings of $20.
Cons: time loss, lower quality light for the purpose (difference in optics), a headache of the potential do-it-all-over-again.