I feel that to make your training or workout safe and beneficial there needs to be a balance of soft and hard training (yin-yang concept). Whether you are a martial artists or not, having balance is good, well at least that's what the eastern world believes.
Most people that are out of balance when it comes to exercising only focuses on the yang or hard (warm up, strength or cardio and technique (combative or art (like dancing, tricking and gymnastic tricks such as backflip, handstand, front level, flashy kicks, pilot freeze, powerswipe etc.)) training, and neglects the soft or yin training (flexibility, cool down and/or meditation or internal training). High level martial artist works with both the yin and yang or soft ad hard into their training, in order to stay in great shape and health.
Having balance is essential for longevity. Eating and sleeping right is also essential for health and longevity and plays important role in balance. Eating and sleeping with the right quantity and quality also contribute to balance as that would be considered the yin or soft part of your training to become the best person (that person is someone who has attained and maintained balance, e.g. great martial artist like grandmasters that are alive today).
Resting is also important when to comes balance as a yin component. When I say rest I mean something other than doing hard training or exercise; basically not working out and taking a break or some time off. Resting helps recover damaged or rebuild muscle and/or bone growth and strength.
Reading or studying during resting period would be wonderful as not only you are working physically but also mentally. (note: there are many type of mental strength which I won't explain in this blog such as inner strength, intelligent, mental discipline etc. In terms of "mentally", which I have stated I am referring to intelligence)
When it comes to training general the yin is like the rest and/or sleep, nutrition, meditation (which can be just stretching) and mental activity (e.g. reading) part of training; whereas the yang would be the hardcore exercise or just exercise (physical) part of training. Within the the physical exercise there is yang and yin in it like cool down or stretching after workout which is yin, and for yang that would be warm up and main exercise routine.
What is the problem with unbalance? having unbalance workout may not be as beneficial as having a balanced workout. For example if you are working out to hard (yang) and not balancing it out enough with some cool down or stretched, then this may result in injury or just next day soreness. The good news is you are feeling pain, and that pain which is your muscles being broken down is good because your muscle can regrow and become stronger. "no pain no gain". Bad news is you will feel tired, weak and painful the next day. You will be susceptible to serious injury which may be chronic and severe if you don't be careful. (best solution obviously is to rest.) This is my comment on next day soreness. As for injury is concerned from working too hard there aren't really any benefit. So next day soreness may be okay but injury is not okay, as a result of yang dominant workout (too hard workout not enough softness). If you injuries yourself too much you won't be able to train which may sound scary, because that means you have to give up and retire. As for working out too soft or not enough hardness, you are not helping yourself grow (grow stronger). This is because since your workout may be too soft you are not challenging yourself and therefore you AREN'T feeling the right pain which is the "broken down muscle" pain, thus you won't grow or improve. Remember "no pain no gain". I like to also say "no stress no success" which is pretty much the same idea but worded differently.
Having a balance of soft and hard into your workout or better into your life style is important for self development relating to health (your body, mind and spirit), longevity, and evolution in advance (if you believe in spiritual evolution).