I maintain squats are dangerous, but not in the way you think I meant. Squats are very high stress, and require a strict discipline in form. The knee is probably the most poorly designed and injury prone joint in the body. Here's what typically happens...
A person decides to do squats. They put some weight on, then proceed. They want to do it right, so they take the weight all the way down to the haunches, way past parallel on the thigh (this means the thigh is parallel to the ground). Nothing wrong- yet. As the person comes back up, the thighs shake, and the knees flex in or out. The person probably doesn't even notice this, so focused on pushing the weight back up.
Now the patellar tendon is very thick, and quite capable of bearing a great deal of stress. The cruciate ligament (inside the knee) is not. Once the emphasis shifts off the patellar tendon and onto the cruciate, you have a potential disaster. Anyone who has ever blown a knee can tell you you won't always see it coming. Pop, it's gone. And once it's gone, it's gone.
So, the more important use for a spotter on squats isn't for support- you're right, the bar can do that- it's to help you watch that form and tell you when it is time to put the weight down. Squats are utterly unforgiving of bad form, and if isn't the knees that blow, the lower back will get you. It is too easy to get permanently hurt to go it alone when you are new and don't know what to watch for.