Mate if the stabiliser muscles are weak, and you go to lift something heavy with your new found strength you will hurt yourself, throw your back out or strain something as you will not have the core strength and stabiliser muscles to support your new found strength.
In fact, according to a recent study by Schwanbeck, overall muscle activation
during free weight Squats is 43% greater than during Smith Squats. This is why you'll build MORE muscle and MORE strength with free weight Squats than with Smith Machine Squats. The difference becomes even greater when using leg presses etc
Train for half an hour with free weights will work every muscle and build stabilisers and core strength building a complete and balanced body with functional strength that you can use in your day to day activities, no need for ab exercises and isolation exercises as these muscles are worked enough during heavy squatting, benching, dead lifting, pressing and rowing, this is not the case with using machines.
Too much use of machines (or lets say incorrect use) will also build the bodies you often see, where people have build the show muscles (pecs and biceps) and they have weak chicken legs, no back and no core strength to speak of, apart from the fact that they look ridiculous, no such issues doing heavy squats, dead lifts, benches, presses and rows, as it will build the body as a whole.
Try doing this after training on machines, you will be in hospital:
[ame]http://www.youtube.com/watch?v=PeZ-xvezhzo[/ame]