Machines don't tend to build core strength and natural affinity for the weight you're slugging around. Last summer I was moving around railway ties(big thick lengths of wood) with ease while the kids working with me were struggling to carry single ones between themselves. One of the struggling kids I was working with used to hit the gym with me quite alot, not a lifting buddy, but we'd always be there together. He used to use machines alot, and do "the circuit" around to all the machines, generally avoiding freeweights except for bicep curls. Meanwhile I was busting ass in the powerrack.
Needless to say he strained his back doing that work while i barely broke a sweat(okay okay i was sweating like a bush pig, but still).
He was no newbie in the gym and he was pretty thick, but when it came time to use the muscle he put on for something other than the prescribed motion to him given by the machines, he got spanked.
There really is no point to this story, but it should speak for itself. Slinging around barbells gives you a "feel" for the weight, and a sense of how to balance a load properly for your body, despite never having lifted it before.