All that has been said about the running threat and step timing and the QB being able to read the defense is valid. Still, you are seeing a lot more shotgun every year. Most of that is because passing is getting easier all the time due to rules adjustments (can't impede the receivers, the QBs are protected, the OL can hold and not get called, etc.). But it's also due to the fact that teams are getting better and better at running out of the shotgun. If your QB can run, then you can have a very good running threat. It's like the old single wing which was a very strong running formation. This still may not be workable in the NFL, where there is such a large disparity between the size of QBs and that of DLs and LBs, but it can certainly work in high school and college.