Mizuki, what's wrong with machines? Of course, I personally prefer not to use them, but there's nothing wrong with using them. They aren't without virtue though, since they almost entirely preclude the possibility for injury. Not to mention, they're great for isolating weaker muscles. For example, if someone has weak triceps, they'll have a hard time working on their chest outside of flyes. Outside of machines, it's very difficult to get a good chest workouts without tricep synergy.
Free weights result in much bigger strength gains, and they also recruit stabilizer muscles.
According to this study, people who did free form exercises had twice as much strength gain as the ones who used fixed weights over the course of 4 months. The ones who did free form exercise saw an improvement in balance/stability 5 times that of the group that used fixed-form machines.
https://www.ncbi.nlm.nih.gov/pubmed/18296958
Overall muscle use in free weight squats was 43% higher than muscle use in Smith machine squats:
https://www.ncbi.nlm.nih.gov/pubmed/19855308
So I think Mizuki is spot-on in saying that free weights/free-form exercise is considerably better than fixed-form machines. Furthermore, if someone has weak triceps, they can strengthen it by continuing to bench press (eventually, the triceps will catch up) and free weight triceps exercises.
That said, I think machines are fine to use if you're just starting out, or if you use them in addition to free weights, but for sigfnicant gains, free weights are absolutely necessary.
Also, just for the sake of argument, I think the squat = more testosterone thing is a myth. If you think about it, your body has no way of delineating between which muscles are worked, and how to regulate testosterone production appropriately in response. That isn't to say that doing legs isn't paramount, but I'd be remiss to agree that leg workouts hold the patent on testosterone production. Superficially, it makes perfect sense, but if you give it a second thought, you have to wonder how the body would be able to tell. It would have to be able to measure the force of the contraction by having special nerve-endings in the legs which would stimulate your jewels into releasing a special amount of hormone.
Recent research shows that the squat = testosterone = greater overall muscle growth may be a myth, but not for the reasons you've posited. The human body is an extremely complex system with hundreds of thousands of signaling pathways and sensory mechanisms. It was previously thought that when muscle was stresed/fatigued/exerted/damaged (as it is during exercise and weightlifting), it would release hormones or signaling proteins that induced the pituitary, adrenals, and gonads to cause the production of various other hormones, including the production of testosterone by the gonads. Larger muscle groups (such as the legs and the glutes) would naturally produce more of these signal peptides/hormones, resulting in a greater production of hormones like testosterone. Other mechanisms, like glucose use/oxygen depletion/recruitment of anaerobic pathways/workload on the heart could also serve as signals to increase productions of certain hormones like testosterone, and again, the legs being a large muscle group require more glucose and use more oxygen and make the heart work harder than other muscle groups. According to the theory, this increase in free testosterone floating around in the bloodstream would cause greater overall muscle growth everywhere in the body.
So in other words, it was an entirely plausible theory, and was based more on the
size of the muscle than some special indication to the brain that "hey, the legs are working out now, better make more testosterone" (not that that is such an absurd idea, and it's totally within the realm of possibility).
But like I said, recent research has shown that that's not what's going on. Yes, any sort of acute activity will produce an increase in overall testosterone/GH/IGF-1, but this spike in hormone levels only lasts for 15-30 minutes. In other words, it's likely not responsible for muscle growth, which is a process that takes days, not half an hour. There's an increase in testosterone, but not for long enough to make any difference in muscle growth.
Recent studies show that the acute spike in testosterone and other hormones does not, for the most part, affect strength gains and muscle growth (
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3371329/). It's now thought that local release of growth factors and peptides and hormones is what triggers muscle growth. In other words, when muscle fibers are exerted and damaged, they release signals to nearby cells and satellite cells, and it's this local cascade of events (see
http://www.skeletalmusclejournal.com/content/1/1/4 for a study that discusses one specific pathway) that causes muscle growth and repair. (Also see
https://www.unm.edu/~lkravitz/Article folder/musclesgrowLK.html for more).
What this means is that working the legs is great for the legs (and any other muscle groups involved), but it doesn't cause some big/medium-term increase in testosterone that'll benefit your whole body. If you want to strengthen legs, work the legs. If you want to strengthen your chest or arms, you have to work the chest and arms--your leg workout won't help them. Doing squats won't leave mounds of testosterone floating around in your body.