Smith Machine Squats are shit. They can take the hamstrings out of the movement. Before you say that's okay since you're trying to focus on the quads anyways, consider that the hamstrings are heavily involved as stabilizers of the knees during this movement. Over time, you're asking for knee problems. It's also biomechanically unnatural for the back being forced to go down in a PERFECTLY STRAIGHT line like that.
I occasionally do Shrugs on a Smith Machine because they involve a very short, simple range of motion. Generally, exercises involving larger ROM's and more complex joint movements are more prone to promoting imbalances, joint wear and eventual injury when performed on Smith Machines.