The problem with a smith machine squat is not so much that you are taking pressure off the quads because you aren' really. The final 10-15% of the movement is still knee extension. The real problem is that you are not efficiently driving through your center of gravity.
When you are performing a real barbell squat you are squating down in such away that allows you to drive straight through your center of gravity. Think if there were a rod running straight from your spine to the ground, it would move in a diagonal fashion with your spine as you squat down. This makes the exercise more functional, increases your balance and strengthens your stabalizing muscles.
When you are performing a smith squat all of those things above go completely out the window. First you are pushing against a fixed bar, which will not improve your balance and coordination. Also, because it is a fixed bar you are doing nothing for your stabilizing musculature. And the biggest difference between barbell and smith sqauts is that in a smith squat you are displacing your center of gravity. You center of gravity is running straight down your spine when you are standing upright. Then when you lean back into the smith bar and keep your feet forward your center of gravity stays up in front of you as your spine and upper body move backwards to the bar. Now you are forced to drive through a false center of gravity which lies infront of your bodies core.
Now if you did smith squats only and never did a barbell squat how well do you think you could efficiently lift things using your bodies natural center of gravity, not only in the gym but in real life? See my point.....it is not functional at all. When do you ever pick things up by sitting back like you do during a smith squat?? Never...you would fall over, right? Also since you are not strengthening your core musculature with the smith squat you are more likely to injure yourself trying to pick something up.
My advice....lay off the smith machine. Grab a barbell and do real squats and your entire body will benefit.