I wouldn't discount these minisubs. The mere possession of a large number of these boats would be a major deterrent. When you get into a shooting war between a submarine and a ship, all that matters is that you have a torpedo in the water. It doesn't matter how you got it there (in an SSN, a typical diesel SS, or one of these miniboats).
For sort of a comparison with the Army, these boats are sort of like a tank cannon that infantry can carry. They deliver an enormous amount of firepower but only with a limited range and speed. But they are cheap so you can buy lots of them.
Thank you burzum for offering some more education on mini subs. A side note, I just finished a novel by Oliver North in which a mini sub were used for an operation. I have also read that mini subs are the choice of a few drug running lords.