Well, it depends to what degree you're training them. Laborers typically aren't the best examples, as most hard-laborers have fairly well recognized long-term musculoskeletal problems (particularly back pain). Gymnasts are perhaps a good example of moderation in daily training. Typically, they don't train at their maximal ability every day - they work on certain skills. While the skills are difficult, they aren't max efforts, unlike most lifting programs. If you're familiar with Starting Strength, you should essentially be hitting a new 5-rep max every time you lift. That takes a bigger toll on your nervous system (for activation) and muscles/skeleton.
Olympic weightlifters are probably a good example of what you can do if you train most days. However, keep in mind that many of these individuals have worked up to training daily with heavy weights over years and years.
Crossfit is probably a good example of how training similar muscle groups days in a row works. Is it effective? Yes. Does it increase you risk for injury? Hell yes. Go to the Crossfit.com forums and look in the injuries section. Crossfit is renowned for causing significant injuries. Is that Crossfit's fault? Not at all. It's the drive of people who are greedy for gains. Sure, they get more coordinated and stronger, but they run on the brink of overtraining all the time. The issue with that is over time, your tissues begin to fail without proper rest. That results in reduced tensile strength in tendons, ligaments, fascia, muscle and reduced compressive strength in bone.
The goal should be to toe the line. If you're on anabolic steroids, then do whatever you want. Your body will recover. If you're like most normal, sensible individuals though, you need to give your body some time to recover. Your aren't just stressing your muscles when you work out, you're stressing your entire body. These things fail over time without proper rest. Eat well, eat a lot, but don't do heavy back squats every day of the week. Taking at least a day off isn't that much to ask for and significantly reduces your risk of injury. Keep in mind that more does not always equal better.