Scenario: Mid-week game in January against a team like Miami or Chicago who is playing the second game of a back-to-back. Their veteran rosters are filled with guys like Bosh/Wade/Deng or Rose/Gasol who want to coast through three quarters to conserve their energy for the long season and playoffs. The Cs open up a modest 6 or 8 point lead in the third quarter, and boom, Stevens subs in this unit:
Smart / Rozier / Bradley / Crowder / Amir

The Celtics start defending their behinds off, pressuring the ball, getting deflections, blocking shots, and generally annoying the @#$% out of the other team. The eight point lead suddenly becomes 18, as Crowder and Bradley rain threes, and Smart and Rozier penetrate the paint and run pick and rolls with Amir. The vets on the other side get frustrated and annoyed and their team mails in the rest game as being not worth the effort.
You gotta admit that would be a mean five-man defensive unit.