Ab and Ad machines are generally junk only because you see women using them solely to try and fix they unsightly stick legs and fat butts.
The Abduct. machine is the only one I use and only when the client has weak abductors. Once I balance them out (i.e., knees stop coming in during deep squats), then they can stop the Ab work.
If you want to increase stability, take a 6 - 8 lb medicine ball, hold it out in front of you with one hand, and do about 6 sets of walking lunges, about 50' per set with about 20 - 30 sec rest between sets. Take a large step and make sure the back knee touches the ground, then push off with the back leg as if you were doing a kick back (butt blaster). Your glutes will hurt for days. I use these in my ab class and everyone bitches about sore glutes in two days. It really increases squatting stability along with the other exercises I mentioned earlier. I use the medicine ball vs dumbbells because holding the ball in front with one hand forces you to keep your shoulders up and whole body balanced. If you drop the ball, you need work. Switch the hand you hold the ball in with each set.
W6