Les Miserables began as a fusion of pop, rock opera, and classical/operetta. As the show has progressed through history, the pop and rock opera elements have been smoothed over, and the show has become modern pop operetta more than anything with a foot in rock.
I saw a production in Pittsburgh last year that used a primarily rock sound in the vocalists, and it worked very well- made the show sound fresh and new because the sound has so often become a vocal replica of the more legit productions.