I don't do cardio in the gym now, so I am always wearing flat stable trainers, however, even though I remind my training buddy 'leg day, wear flat shoes', he sometimes forgets.
I can see the instability, his heels shifting from one side to the other at odd angles for squats, and that soft heel shifting travels up his shins to his knees during leg press.
It really is a bit dodgy, and personally, one thing that really screams 'I don't have the foggiest how to train' is seeing some lad squat in Nike Shox.