Some would say the forms/katas are not useful. I agree and I disagree. The forms are useful in they are helpful when an isntructor wants a way to warm up a class or a student wants to warm up muscles, get the blood flowing before a class begins.
Forms also are a good tool to use when a student or instructor wants to perfect a certain kick, punch, stance or block. The more the form is executed, the better the artist becomes at executing that kick, punch, stance, hand strike or block.
However, trying to use an entire form in a combat situation is useless, unless there is a person at every spot in the form for the artist to be successful in completing it by the numbers. Even then, it might not work as planned. The movements probably will have to change because no attacker[s] will stand exactly where s/he/they are to stand in the artist's mind as the form is executed, causing the artist to extend a stance or try to stretch a punch, strike or kick or change a block because the attacker[s] was/were out of range.
Some of the forms used in dojos are not explained, just done. Some of them have steps that seem silly, even if there is an application to match the silliness. Some instructors even go so far as to tell students they do not know exactly why certain parts of a form are there, just it is part of the form. My question is. . .Why use it? If the instructor cannot give a sensible answer as to why the parts are there, they should not, in my opinion, introduce the form to any students until they can provide a rational answer for its use in the training program. If it doesn't make sense in the training program, chances are it will not make sense in a real combat situation.
Adding to the disagree side, the forms are misleading because they only offer expertise against an imagenary attacker, causing, a good deal of the time, anguish, even joint problems for many artists, over the long haul, especially all the kicking and punching at the wind, hyperextending the kicking legs and punching arms. This is done because the students do try to put power into the punches and kicks, not realizing the anguish that will come after many air kicks and punches that do not grab a target at all.
The instructors who realize what might result from years, maybe not years, of kicking at air will not allow students to air kick but use that force against some sort of focus target [i.e., a shield, bag, mitt, pad].
A lot of this training comes from traditional teaching. That is to say, what and how an instructor was taught, the instructor will teach to her/his students.
The dojo is a place for traing against actual combat; the streets is a place for using that training. In the streets, the situation is, more often than not, critical. And, most of the time, there is no room to think about which form to use or which of the parts of a form to use in a given situation. There only is time to react in a way that is most advantageous for the artist.
Having said that, I do think some of the forms/katas do look good, when executed correctly, regardless of how slow or fast they are executed.
If your main goal is to train forms for demonstrations and testing, follow the philosophy described below. . .