I'd say there is a difference between a booklight and just shooting kinos through diffusion. It's not all about the size of the source relative to the subject: specularity, falloff and "spread" are also key concepts. I think booklights are very good at killing specularity, and, as previously said, reducing falloff. You can use a lighter diffusion on a booklight as the primary source is already bounced and less specular.
Depending on the diffusion you are using, booklights can give a very pleasing, natural look. I've seen them being used extensively on fashion/beauty shoots.
Booklights are especially useful for very large frames, at the condition that electrical power isn't an issue and when you have enough room to do it properly. This technique is also very useful for exteriors, as you can easily booklight the sun itself and get a much softer look than by just diffusing it (although in this case, double diffusing also works).
However, I would agree that, on a fast-paced set, setting up a booklight can be considered too time-consuming given the slight difference with other techniques. You mentioned Kinos; it's a good idea, I would shoot them through Depron in order to get a look that's closer to a booklight.
Now, depending on the surface you're bouncing the primary source into, it cas be more or less specular. I like to play with specularity. Controlled soft specular reflections can be really beautiful... But I already talked too much! I'm too young to reveal the tricks I had such a hard time discovering!! Forget what I just said!!
Edited by Guillaume Cottin, 24 April 2013 - 07:14 AM.