This is certainly one of those topics that has been written about ad nauseum, the best/worst part about the debate about the efficacy of kumite is that there are so many points of view on the subject. The interesting part is that the debate isn't concerned with traditional versus competition type kumite, or style versus style even; the argument, such as it is, is about whether kumite is necessary or not. I find this concept to be laughable, mainly because it seems so obvious to me that when you're learning a martial art, fighting would be a more than necessary facet of the training.
For the sake of addressing that argument, let's just go along the basis of whether or not kumite training is necessary for martial arts training. There are those that might contend that the true purpose of training in a martil art is character building, or cultivating self-discipline. These are wonderful reasons to train in a martial art, but frankly you can develop a very strong character through playing any team sport, the type of coach you have will largely dictate how much character development goes on - the same can be said for karate training. You can, and should, learn self-discipline from your parents, at the end of the day discipline isn't actively taught; it's passively learned through observation. The role models children have in their lives are the ones that show how to be disciplined. This is something that can be seen in high-level karate practitioners, but it really should be learned growing up in one's home. Since we've now realized that you can gain these benefits without martial arts training, let's look at the one aspect of martial training that really can't be taught (safely) anywhere else. Plain and simple, that's how to fight! Martial arts without fighting is simply aerobics training. There's nothing inherently wrong with aerobic training, but just don't call it a martial art.
Now that we've got that out of the way, let's dig into the real question; what kind of kumite training is beneficial, what is less beneficial and what is plain useless. Let's do this backwards, and start with the useless stuff: no-touch sparring and one-step sparring. Before I'm turned into the pariah of the karate world, let me preface by saying that these aren't completely useless from the point of view of learning mechanics and as light training, but for an advanced karate-ka or advanced fighter, this is just not useful outside of anything but a warm-up situation. What is the purpose of sparring? Pressure testing technique and maneuvers against a fully resisting opponent to figure out what you can make work and what you can't. Naturally we aren't trying to kill our training partners, but we do need to know, as fighters, if we can deliver strong technique effectively. The other draw back of no-touch sparring is that one will never experience the feeling of getting hit in the face. I know that the best defense is "no be there," but what if you aren't fast enough once? That's all it takes! Learn what it feels like to take a hit, and learn how to survive taking the hit. That's one of the greatest benefits of hard contact sparring.
One-step sparring is a semi-effective training tool that is unfortunately taken much too far to be truly effective in training fighting technique. The idea of a single attack and single defense is nice, but frankly after throwing the block, there will rarely be sufficient time to throw an effective counter attack. One-step sparring, in my opinion, is most effective when used to train intercepting attacks and limb destruction. Don't be that instructor that strings along 3+ techniques because it looks awesome and claim it's an effective combat skill. Be up front with your students and tell them that it's just a fun lesson in mechanics that will be largely useless from a self-defense context after the second technique.
Enough bashing on traditional training methodologies, let's move on to the less beneficial stuff. That's the WKF and most open-style tournament sparring rule sets. The nature of the game, and it is a game, encourages fighters to move in such a way as to be completely vulnerable to a truly aggressive attack from anything outside a karate/taekwando/kung fu syllabus. The average karate competitor might claim that bouncing like a rabbit keeps him mobile because he's light no his feet, but there's a big difference between being light on one's feet and being airborne. Watch most MMA fighters or kickboxers, they are incredibly mobile but they rarely, if ever, leave the ground. That being said, there is some benefit to be garnered from WKF type competition; timing, linear speed, foot work and speed. Reaction timing and explosive ability are the skilsl that will carry over into a real fight, the rest is largely fluff.
Now onto the meat; full- or semi full-contact sparring. Shobu ippon falls somewhere along the spectrum between beneficial and less than beneficial; hard contact is encouraged, and fighters aren't overly bouncy. However, there is a lot of focus on the single technique, which isn't a bad thing but it's far from the be-all-end-all; sometimes a long flurry of punches is called over to end a fight. This kind of hard-contact sparring is great, but not quite as good as the kind of sparring you'd see in muay Thai and Kyokushin sparring. The purists are already rolling there eyes and whining "but that's all sport! We're training for self-defense!" That's all well and good, go and get your head caved in. There is no self-defense, there is self protection and situational awareness. Self-defense is when you're surprised, if you're training for that then you're training for the impossible. If you're surprised then your reactions will be visceral, no matter how much you train that head block-reverse punch combination, if somebody comes at you with a knife, that will be the last thing on your mind. Situational awareness gives one the opportunity to fight for their lives, or avoid getting into the situation requiring it.
I will always encourage any person to take up Shotokan, I love it and the benefits that I mentioned earlier are there; if you find the right dojo you will develop strong character and self-discipline, through your own hardwork and observations. That being said, even if your dojo doesn't encourage it, I urge you to slap n some boxing gloves and head gear and go at it with your friends and training partners. Your character, self-discipline and your confidence will thank you...your body may not right away, but it will. I promise you.