Ah....what lady said she was trying to target her quads....that didn't really make any sense to me.
Like Adam said, the plates sort of raise your heel like shoe. That will put a little more stress on the quads. Most people I see using the plates are doing front squats. Having a heeled shoe is nice for doing that lift. When it comes to regular squats, some people like a heel and others like to squat in flat shoes. The downside to the plates is that they could slip, and that could get ugly.
It seems to me that whatever minimal quad benefit is derived from this practice isn't worth the risks associated with it. Just squatting seems like it should be enough.
Unless there was some device that could attach safely to your foot and give you a little bit of heel raise relative to the toes, that would be designed specifically to allow you to better apply force between the floor and the barbell and even give you a firmer, better footing than you'd have otherwise...