Lets say you're someone that wants to burn fat and pack on some muscle, are cardio machines any good for that? I would say not really... Many trainers (good ones anyway!) have moved on from cardio machines to weight training. Think about it, if you want to gain muscle, then weights are your friend, and by placing your body under stress because of the added weights, that will burn more calories.
You do still get trainers that 'plonk' their clients on a treadmill for 20 minutes and leave them, those are the people you should NOT hire, firstly it's not 'personal training' and also, it sucks! If you are a female, you may be worried about adding muscle (if you don't want muscle mass) if you train with weights, well, you're wrong. You aren't going to turn into a mini 'Arnie' if you start using weights. When I say weight training, I'm not talking about the training the 'beasts' in the gym do, I'm talking about light weights. There are plenty of body movement exercises right? well, they are good, and if you do circuits of those, that will burn fat. How about adding weights into those exercises? it will simply make you work harder, therefore burning more calories. It shouldn't be heavy weights unless you want to go into proper weight training.
Does that mean you shouldn't use cardio machines at all? no.
I personally use an exercise bike for about 5-10 minutes to simply get the blood flowing. Then I perform a little circuit to get myself pumping. Some people will say to me 'that is a waste of time, what are you doing?', well, it works for me. A fair few guys that I know that are in good shape use a cardio machine as a warm up, but that's it really.
If you want to train for endurance, then yes, you need to use machines or your own body (i.e. a run). If you want to run a marathon, then you need to run, simple as that. It really does depend on your goals, but if your goal is for fat loss, I would definitely recommend you use weights instead of going for a brisk walk on a treadmill.
The great thing about using weights instead of cardio machines for fat loss, is that using weights will actually help tone your muscles - giving your body shape. Running on a treadmill does burn calories, but not as much, and it doesn't really give your body a good shape, whereas weights do.
Cardio machines do have their benefits though, especially with the elderly and those coming back from injury. Its quite clear than elderly people can't put themselves through a weighted circuit (I suppose some can...), and when you're older, its very beneficial to keep moving and getting regular exercise. With coming back from injury, I remember when I had a thigh/hip problem, and I was out for quite a while, and to start with my physio got me on a treadmill to get back to walking, then jogging, and then full running, so it was beneficial.
It is common for people to just hop on a treadmill, as its easier than having a circuit in mind using weights. Don't be lazy, go to the gym to train! not jog!