Without commenting on what Debian should do about this, I've pretty much given up using `which` myself.
Since `which` is an external command (it's built into some shells, but not bash), it doesn't know about aliases, shell functions, or builtin shell commands.
I've found that bash's built-in `type` command (with its various options) does whatever `which` does, and often does it better.
I also use `command -v foo >/dev/null` to detect whether the command `foo` exists -- for example:
if command -v less >/dev/null ; then
export PAGER=less
fi
It would be nice if `type` and/or `command` had an option to check whether a command exists without printing anything, but having to add `>/dev/null` is only a minor annoyance.