Are long tones useful? The predominant tendency, while holding a long tone, would usually be to increase playing pressure, tighten the embouchure, or unconsciously activate various improper actions as one continues to hold the tone. If one can manage to maintain proper ease of playing pressure, etc., it may be worthwhile, yet I really cannot see what value there would be in doing that. I cannot see how it is of any value to hold the tone for an extended time. I seems to me that the longer you hold the tone, the more would be the tendency to re-activate any bad habits, or wrong actions based on fatigue or boredom. And consider……why….for what purpose…..would you want to use long tones? There has to be a reason.
Rather, I suggest producing intelligently thought-out, beautiful, and musically-enriched tone, to be played in relatively short lengths, perhaps as briefly as 1 to 3 seconds, then repeated, giving attention to proper tonguing and breathing while so doing.
If your goal is to improve your tone, properly done lip slurs are the answer…not long tones.