IMHO, the argument boils down to representing and passing on the art. A hobbyist practicing bits and pieces, represents little harm. However, practicing 50%, passing that knowledge on, misrepresents the art. You end up with people who trained 100% of what they were taught, and still only know 50% of the art. Many of the folks on this forum have devoted a large portion of time and energy into training their representative arts, and have taken on the responsibility of being "keepers of the way." Deviating and leaving those parts behind, hinders your ability to pass on the art. I.e. it only enables you to pass on the parts you maintain proficiency and knowledge in.
How long does it take to lose proficiency in your forms training? Not long, a couple of months off of consistent training, you may lose small details. I see it in my college aged students who come back to train in the summers. Some practice while away at school, others do not. Those who do not, have to struggle a bit to knock the proverbial rust off. Imagine if you took years off, and then decided to teach others?
Some one who knows how to stick weld, and practices stick welding, but doesn't know how to mig or tig weld, would you call them a welder? I don't think any certified welder would think so.