I guess it depends on what your definition of "spin training" is...it's not all created equal. This can range from immediate recovery from the incipient spin, to recovery after 1 or 2 turns, to full-blown flat/accelerated/crossover spins in both upright and inverted modes. The latter obviously will not be practical in most RV's due to the need for inverted systems.
Most spin training involves plain-vanilla upright spins in Citabria/Cessna types. For this limited type of spin training, spinning past 2 turns serves little purpose other than to get your inner ear in a potentially vertigo-inducing state after the spin has been stopped. This can be valuable in and of itself, but it's not typically the point of your average spin training...and it's rare for an instructor to have the student perform the number of revolutions needed to really get your inner ear going anyway. Try a 12-turn spin sometime and you'll see what I mean.
So if all you're doing is basic upright spins, going past 2 turns will not affect how the proper spin recovery inputs get imprinted into your brain.
So for "typical" spin training, I don't see why an RV-6 could not be used, as long as your instructor is familiar and comfortable in type, and turns are limited to 2-turns or so (only to prevent unnecessary stress for the student, and to more closely replicate more "normal" spin characteristics). I have never spun a -6, but know experienced aerobatic pilots who have, and they all say that there's nothing funny about them other than the fact that they rotate quicker than many other planes once fully developed, and take a little longer (once developed) to recover than the FAA requirements for TC'd airplanes that are approved for spins.
Unfortunately, it seems this Vans
recommendation to avoid spins in the -6 leads to apprehension about using a -6 for aerobatics, especially competition, given that a spin is required in the most basic category. But competition spins never exceed 1 1/2 turns, so this fully-developed state would never be experienced. But then if you are inexperienced or uncomfortable with spins in your airplane, it's not a good idea to be doing aerobatics in the first place. There's a lot of fear to begin with on the subject of spins, and given that most pilots are not comfortable with, nor have extensive experience with spins, Van's recommendation is probably a good one for the average pilot, on their own, not in a structured training environment with an experienced and qualified instructor.
That being said, since there's nothing unsafe about spinning a -6, to avoid them is to acknowledge that there's a gap in your experience/comfort level in your airplane throughout its performance envelope. This alone would motivate me to get the training needed to become comfortable and competant enough to explore the full flight envelope. For the aerobatic pilot, advanced spin training as first mentioned above (inverted, etc.) is extremely valuable. After this type of training, it's highly unlikely you'd bat an eye at spinning your -6 any number of revolutions.