I don't think that it's guarded. UTD has instructional videos that can be purchased as well as an online classrooms, without going to the instructor first.
For example:
Essentials DVD
Essentials Online Class
That said, many of us have found that instructors do make a difference in teaching any physical skill (not specific to scuba). And learning incorrectly is often more frustrating than not learning at all (Law of Primacy). If you read enough "what should I practice before Fundies or Essentials" threads, you'll see multiple discussions of breaking bad habits.
Specifically to scuba, since we have limited space and time to practice, it often makes sense to practice correctly from the beginning. In addition, having an instructor demonstrate, teach, and then critique technique quickly ramps the learning. Instead of spending 5-10 dives trying to understand the backwards kick, a good instructor will be able to shorten the time significantly.
However, if you feel like you have good motor skills, willing to spend the time, and then get fine tuned by an instructor, that can definitely be very effective. It's just a longer road, more trial and error, and you have to be willing to relearn.