Neat idea but i don't know how useful it will be in real life situations. In an angry mob there are hundreds of people (its not called a mob for fun!). If someone is being attacked, people resort to human instinct (protect your children and then every man for himself). They are not going to stand there sending new waves to the vanguard as the previous ones return to the back (that would be a good strategy on their part seeing that by the time the initial group gets sent to the front again, they'd have completed the "cooling period"). Most likely a few in the front will be kept in the front and will receive blisters if not burns. A better crowd-control would be to use that linear sound wave device they showed on Future Weapons.

Btw, i loved that diagram of how it works(the skin part was actually a little informative).

Truck -----(beam)--------->[person]

This could be awesome for use as an actual weapon though. I'm sure they could tweak it so that it's waves penetrate deeper into the skin or just so that its much more efficient at heating things up. Either way, it'd probably be great for making a grilled cheese sandwich.