We have seen Waters throw it, and while he is a good passer, imo, he also has a high amt of interceptions.
Also, we have seen waters run and it is the absolute worst.
Sams running = Ell
Sams throwing simply can't be bad enough to overshadow his running ability so that the two don't equate to more than the sum of Waters' running/throwing. Also, we have seen him do it well.