This comes up every 6 months or so and inevitably devolves into a huge
flame war. The general consensus seems to be:
1. /bin/sh must be a POSIX compliant shell.
2. Any script that says "#!/bin/sh" but uses bash-specific constructs
has a bug in it.
3. /bin/bash must be present.
(The third item seems to be the one that annoys the most people.)
As far as I know, no one has replaced /bin/sh with something besides
bash (ash, for example) *and* reported back sufficiently positive
results to allow us to change the default /bin/sh to something smaller
than bash.