Personally I think you can train everyday so long as you understand that training takes many forms and all forms have value and can be deemed productive. Too often training is only thought of as "strength work" or "conditioning work" or "metabolic work" and that in order for a workout to be productive it must challenge the body to new heights. But what about the value of improving joint mobility or getting the blood flowing or just practicing an exercise for the intrinsic value it brings. All are benefits of exercise.
We can't view training in such a one dimensional fashion. If one was to train everyday using the same modality or style of training (strength training) for the same purpose (to gain strength) than a problem would come up. But if one was to train one day for strength development and the next day used exercises to "limber up" and get blood flowing and followed that up the next day with a "metabolic" type workout....etc and continued for the majority of the week cycling between intense, moderate, and light physical activity there should not be a problem. Very much like academics, if one were to push hard in one subject all the time, that could lead to burn out, but if they used one day to learn new material and the next to review old or even switch to a different subject then the goal of gaining knowledge and receiving an education would be met.
If we (experienced trainees who don't have other physical activities) viewed our training like a sport and the different venues of fitness as skills which needed to be developed than training each day would be valuable (much like sports practice). And just like athletics there would need to be hard days and easy days as well as rest days. I don't really like that whole line of thinking that anything enjoyable is recreation....If you don't enjoy your training why do it? Training should be enjoyable and rewarding.....not painful or monotonous. A productive exercise / workout is one that produces a result. A strength training workout performed with a high level of effort will cause a result. A less intense movement workout the next day will cause a result. Namely bringing in blood and nutrients to the muscles....aiding in recovery.
Unless you are a [true] bodybuilder where your only concern is "growing", training to the point where you can't move or "function" doesn't make sense to me anymore (even if that is deemed as the "best" way to train). This is something I have learned working in athletics. If I train someone so hard that they are sore and "heavy legged" and can't practice well.....what was the point of training? Training is supposed to compliment your activity not compete with it. Granted, I no longer play organized sports but I do have to move around at work and "survive" long days training people so training that way is not needed and somewhat counter productive. -
Douglas Scott, The Pingry School, Strength and Conditioning Coach