Originally, rm would merrily nuke your whole filesystem if you told it to. At some point, someone thought that was a pretty stupid default behaviour, so they added that flag to change the default to not nuke your entire filesystem. However, they made the change backwards compatible in case someone still needed the old behaviour. I can imagine in a container or throwaway environment, it might be vaguely reasonable to expect to be able the blat /.

See also:

Unix gives you just enough rope to hang yourself -- and then a couple of more feet, just to be sure.

-- Eric Allman