It's not a hack; it's simple argument validation in order to improve usability.  It saves users from the very easy mistake of including specs in their `spring test` invocation via an over-zealous glob or a misconfigured `Guardfile`.  If that mistake wasn't so disruptive then this wouldn't be needed, but the fact that including even a single spec prevents *all* the test-unit tests from running caused me to waste about 2 hours trying to figure out why they weren't running.  I really don't want other users to go through the same suffering.