The squat and the wall-sit both are done for different purposes. You could mix them, but that could mean diluting the individual advantages of each. Try doing a wall-less sit of the kind you describe, you will feel very different pain in your muscles compared to a wall-sit. Also, the time you can hold each will be different. That should be a signal that the body mechanics are quite different.
A wall-sit is done primarily to strengthen the quadriceps, isolating them from other muscles. The straight back helps in the isolation, so that the glutes don't come into play. By maintaining the 90-90, my personal observation/feeling is that the hamstrings are also not worked. This isolation helps to build the quads, or rehabilitate them after injury. Since a single muscle is worked and the joints are stationary, you will generally feel an intense burning feeling in the thigh. This is the quads getting worked.
The squat position is more compound, it works the glutes, hamstrings, hip adductors, and the quads too, though not primarily.
In conclusion, if your goal involves massive, strong quads (think cycling, horse-riding, skiing), or you are recovering from an injury, the wall is your friend. For general fitness, a compound form, especially with movement (squat) is more balanced and has greater benefits.