If cantering is bad for a gaited horse, my mare is screwed.
We canter all the time. We even do jumping courses and flat work that works the canter.
(Walkers I've ridden have lovely canters too. Rackers/Pacers like mine seem to need a little extra canter training, but smooth out with practice.)
Cantering builds wind and endurance. It exercises different sets of muscles. Galloping is excellent for working out soreness. Plus, most gaited horses love to canter. Many people only work two of their gaits -- walk and easy gait. Monotony breaking is good.
My mare has four gaits we work: walk, trot, canter, and her saddle gait. The more I work her at the trot and canter, the better her saddle gait has become. Working all her gaits has made her stronger and SMOOTHER.
So yeah... I'm a big supporter of cantering and working gaited horses in all their natural gaits.