Although I frankly do not use Cycles much, I often render without shadows, then inject the shadows that I want using “shadow-only spotlights.” (The shadow information is captured separately so that I can adjust them further with compositing: once the spotlight tells me where the shadow is, I can control its density and tint.)
Shadow calculations are a big time-waster in (at least, BI …) rendering, and, while the shadows will of course be “exactly correct,” sometimes that’s a much harder situation to deal with than one in which the shadows are only “where and how I want them.” I can simplify the shadows while still keeping them at-least plausible … and save a lot of time.
In “real world” situations where I would have to dance-around with lights to eliminate unwanted shadows, in the magic-world of 3D I can simply choose not to generate them. Sweet!
Even if you’re using Cycles to do most of the work, you can also use BI’s shadow-spots (and/or anything else that BI offers) to provide supplementary information for final image-assembly which is done using compositing.