It's easy to see the appeal of gymnastics training. It usually requires little or no equipment - mostly bodyweight exercises and maybe a set of rings or paralettes - builds impressive physiques (check out the body on any high level gymnast) and looks impressive as hell. Watch someone knock out a handstand pushup or hold a full planche and you can't help but be wowed, even if you aren't an athlete yourself.
There are roughly three reasons a martial artists should or could incorporate gymnastics moves into a strength routine:
- Build strength in a movement to balance force production around a joint (for injury prevention). For example, you should so some kind of pullup or body row regularly because most martial artists do a lot of pushing (pushups, punches, etc.) and if your strength in one direction is too much greater than the other direction you set yourself up for injury (in this case, at the shoulder joint).
- For functional/ tactical reasons. The best example of this is the muscle-up (start hanging from a bar or rings; pull up high enough that you end up above the bar or rings, like the top of a dip). The movement in a muscle-up is a lot like what you have to do to climb over a wall or ledge or high fence. This has clear self defense implications - imagine you're trying to escape from some attackers and come up on a low wall or fence. Strength developed from muscle-ups could help you get over that wall or fence very quickly to escape.
- For performance vanity. Performance vanity is my term for feeling proud of yourself not for looking good but for being able to do something you think is cool. I'm sure we've all felt this - the first time you made your gi snap while kicking, your first knuckle pushup, your first board break. There's nothing wrong with this - I'm not against vanity as long as it motivates you to do good things (vanity that makes you exercise and eat right is good; vanity that makes you shoot botox into your head is bad). But you should recognize why you're doing what you're doing.
Look, if mastering some new, esoteric movement jazzes you up and gets you excited about training (that's performance vanity), then there's nothing wrong with that. But if your training routine is already full and you're out of time to add to it don't feel bad about skipping the work on pistols, handstands, and planches in favor of movements that directly improve your art.
Let me dissect the pistol for another good example of this. The pistol is a one legged squat - you hold one leg straight out in front of you and squat down until your butt touches your heel, then stand back up. Fantastic exercise, right? Most people can't do one. I'm sure it impresses people at the beach. But why can't most people do one? It's because of either their balance or strength down in the hole - in the bottom position where the hip and knee are both fully flexed.
If you want to be able to do a pistol you can add a bunch of work in a deep squat - building up your strength in that bottom position - and spend time developing your balance in that position. But I doubt you ever have to get into that position in your art - I know I don't. Even if you do, most of us have to do our pistols so slowly that the strength won't carry over well to fighting speed. You'd be better off doing deep two legged squats and working at a faster speed - you'll get better carryover.
Take home message: practice gymnastics moves or weird bodyweight exercises if it gets you motivated to train (I do this myself). But don't sacrifice the kind of work that will directly improve your art for the sake of being able to do flashy stuff that's peripheral to your karate.
More caveats: I don't mean to disparage people who incorporate gymnastics into their martial art - who do tricking or extreme martial arts. I love watching that sort of stuff - I don't do it myself and I object to people who think that it makes for good self defense, but that's another story. If you do practice that then by all means gymnastics should take a central role in your training!
Let me say also that I have the highest respect for gymnasts - they're fantastic athletes, and if you're interested in a physical activity other than martial arts you could certainly do a lot worse. In fact, if you want to be fit and don't want to do martial arts you should seriously consider a program of mastering gymnastics moves. Just remember the whole thing about chasing two rabbits...