Back to the original topic.
"Kata" is one of those things that it's not just ONE thing, so how it is being defined by the people in the discussion is crucial.
In its most basic form (no pun intended), "kata" just means form/pattern.
There are many two person katas in the martial arts, so it is not just "solo practice". For example, Judo's self-defense (Goshen) techniques that Kano thought too dangerous to try in randori were practiced as two person drills. Kendo has a series of katas that involve and attack and response between two people.
There are katas that are very short in length. Take, for example, iaido. They are a solo practice that involve drawing the sword, making a cut (or multiple cuts and/or a defensive move and then a cut or cuts), then cleaning the blood of the blade and then putting the sword back in the scabbard. All of them are based on these 4 basic ideas.
So, under this more "broad" definition of "kata" we can say that any prearranged pattern used for training is a kata. For example, if a boxing trainer is having his student go through the same prearranged combination over and over in the air and even on a pad it could be considered kata practice. Even if we add a partner and have them throw a specific punch and respond with a specific defensive maneuver followed by a specific offensive response, their method of drilling (to use the western term) is the same as kata practice in the asian cultures that used kata.
Now some define "kata" more narrowly and ONLY define it as the predefined patterns that are longer in length used by Asian martial arts for solo practice.
Next, we run into the other schools of thought in regard to kata and its usefulness. First, the people that understand that kata has to be pulled apart, practiced and drilled in increasing resistance to make it functional for fighting and also understand that it is a way to teach you how to move your body in a certain way based on how the system is set up. Then, there is the school of thought that markets itself to people who really don't want the bumps and bruises associated with fighting and imply that through solo practice alone of kata they will become an unbeatable fighter.
SO....is kata useful? Depends, it is useful if you find it useful and congruent with your goals in martial arts. It is not useful if you don't see the value in it and it is not congruent with your goals in martial arts.
Do you need kata to learn how to fight? No, you don't. But, that also doesn't make them not useful. I can pull out drills, techniques, tactics and strategies that are easy to use and distill them into a "fighting system" that would be very easy to learn without the need to learn the entire kata/system. But, not everyone wants that, so this goes back to the previous statement about what are your goals in martial arts and is the path your are taking congruent with those goals?
