Well, not necessarily. A Tanto is smaller and easier to wield as it lacks the length and, subsequently, extra weight of a katana. It allows you to be more fluid and "flexibe" in your moves in exchange for longer range. And the spin doesn't necessarily leave you open to attack either. If I use this in conjunction with freeform taijutsu, I leave the enemy little time to draw a weapon to counter or attack me with a feasible counter. Imagine if I am dealing taijutsu blows to you, and after throwing a strong kick to your abdomen with my right leg, I pivot on my left foot and unsheathe the tanto with the right, aiming for his side. I'd strike him very quickly after my foot makes contact. It's all about how you use the jutsus in conjunction with others.Wait a tanto? The size of that sword alone puts you at a huge disadvantage. First of all I would need to get into serious close range, almost on contact type range. If an opponent was to have a normal sword all he would need to do is take it out and like that I am stabbed. Personally this attack does not fit to my liking and will leave me open to much, another serious problem is the user is "spinning" to try and attack the opponent. Well that means while spinning I will be open to back shots. The opponent can easily attack me while I'm just spinning like a mad man. Also I doubt I would be able to notice my surroundings while using this attack.
The only good thing to this attack is well its a simple attack and can be executed at any time.
Good, but why not intercept the arm before it lands? As they spin, why not just grab the forearm of the enemy?Interesting question >_>
How would I counter this ?.... Well as you mentioned this can be used for fluid and control movement. There speed will be much more quicker then mine. So if I was to notice my opponent to pull out a small sword like a tanto, I would simply try to stay away from him and keep a certain distance but by using my longer sword. The reach of my sword would make him unable to get close to me when at the same time I would parry and attack he had due to my sword being much longer and stronger.