Usually the only time it is used, is when a wreck is found by sidescan, in water too deep to dive it by normal air or Nitrox, and they need verification/provanance of the wreck before putting any more work into it. Then it might be used to work the wreck also. Usuallly Tech diving is used in deep caves, and water deeper than 180' feet deep for long periods, or anywhere that needs deep penetrations into a ships hull at those depths. The reason why it is not easily practical and common is all the cost of gases and equipment, and the maintainance of it, and the extremely long decompression times. I do know of quite a few shipwrecks that barney (Mike B) here on the board has verified. he has also been on the History Channel a few times doing so.