Some times ago I had a bad experience with the libreadline package. The installation failed because a required package wasn't installed. That's ok, but the libreadline package had already replaced my bash with another one which wasn't working anymore. The bash couldn't be executed because the dynamically linked libreadline wasn't installed.
Since I wasn't able to login anymore, I booted the system with a boot disk and replaced the symbolic link bash -> sh with tcsh -> sh. It was still not possible to boot because at least one important startup script (rcS) contained #!/bin/bash instead of #!/bin/sh (I realized this later after installing a new base package and hours of work). I reported the problem with libreadline to the package maintainer (and the bug list) without getting a response. I would really aprecciate if the debian base system uses in all important system scripts /bin/sh. It would be also safer to use for sh not a symbolic link to bash but instead to ash or another bourne compatible shell to avoid problems if the bash is broken. -Werner
