This tactic was used and written about quite extensively by Romans and neighboring kingdoms during the times of sword-and-shield warfare. Although modern warfare is dominated by gun and artillery, would this technique be useful in modern-day conflicts (I.E. riots)?
It is actually used by riot officers as a way of protecting themselves whilst advancing or retreating.
http://www.businessinsider.com/kiev-riot-police-use-roman-tactics-2014-1/?IR=T&r=SG
This recent example in Kiev is one example. The are many others too.
It would not be useful in other situations though, as shields that can protect from bullets would be unfeasibly heavy to carry above your head, and a pain to carry around in combat in general.