I know of a story where a guy was laying on a bench holding two heavy kettle bells over head when his dog jumped up on his belly.
He tried to set things down properly but ended up pulling some muscles.
My point is that using weights is a great way to use all of your muscles but you have to be careful.
You need a lot of concentration to exercise properly and effectively.
Let's look at some reasons to use free weights of any type: A)   They are extremely versatile.
You can literally do hundreds of exercises that work every muscle group in the body.
Machines typically only work a specific muscle directly.
B)   You have a lot more freedom to move with free weights.
With machines you are limited to the motion of the machines ability where as free weights allow you to move to your motion ability.
C)   One of the best reasons to use them is that they allow you to work several muscles in one exercise movement.
When using these weights, you are able to use the smaller muscles you would not be able to work using a machine.
Imagine doing a bench press with a machine.
You're basically pushing the bar up and letting it down.
You also have to stabilize the bar in all directions lowering it down slow enough and stop so you don't crush your rib cage.
As you can see there are is a lot more involved with free weights.
Now I seem to have come off with a strong bias towards using free weights but machines do have their place.
I think it is best to use both for a truly powerful workout.
So don't be afraid to use the weight machines.