Long tones require only one task and that is to play a single note. There's no need to spend mental energies on articulation, flexibility or anything else besides tone production. This is great when it's used as a rudiment. It helps us see things in our playing which we cannot see in a more active context. But the fact is, real music is not isolated to one physical aspect.
Music requires a complex combination of all the physical elements.
And while isolating individual aspects helps us to analyze specific problems in our physical playing, it is not enough, on it's own, to help us learn how to balance ALL of the physical aspects in order to create real music. Of course, the best way to learn how to balance all of those physical aspects is to practice music, real music, not just exercises. Playing real musical phrases and melodies brings all of the physical attributes of our playing into one place and gives it a purpose. I've done all of these things individually. Now let's see what happens when I put it all together.