Well Both Sides have Merit..

"If you are training primarily for street self-defense, I don't feel it is very important."

Wow, I like to read alot of your thoughts Mr. Miller, but I read this and shuddered. I think its this exact thought process that has made alot of people overconfident in their abilities and placed them in extreme danger if they run into a real problem on the streets.

I believe strongly in sparring, in many different scenarios, 1 vs. 1, 1 vs. 2, 1 vs. multiple, only stand up, only clinch, only ground, all ranges, vs. weapon, etc.

I think that some of the most important benefits of unstructured combat(freestyle sparring) vs. structured self defense techniques on the body, is the ability to deal with adrenaline, surprise, and the fight or flight reflex. The long term ability of recognizing how the opponent positions and primes his body to throw certain strikes so there is recognition in a real situation, and of course a feel of contact that may not be expected. There are obviously many more as you stated in your post, but I feel that sparring is an integral part of training.

That all being said, the original post was in regards to higher degree black belts.
Thats alot trickier, I stand by the statement that sparring will help anyone at any level be better at self defense, but that obviously depends on the physical health of the person training.
Lets face it, those that are higher degree Black Belts, legitimately anyways, are generally getting up there in years, some have taken better care of their bodies, and some have not been so fortunate. Obviously if sparring is going to put someone at great risk for personal injury then I see no reason to risk it, the rewards are outweighed by the risks.

I think Kenpo Karate is an art that requires a practitioner to enjoy hard or rough contact to be able to achieve Black Belt let alone a high level of Black Belt. I would Imagine that any legitimate high Level Black Belt has put in an insane amount of time sparring, or freestyle combat, whatever your take is on it. I think most have a good sense of how their Adrenaline works, how they react to contact, and how the body works. I think that much like Riding a Bike, if you have done an extensive amount of sparring over an extended period then you retain a majority of the benefits from it. I am sure there will be rust, and things like Timing will be a bit off from where it would be with more current training, but I think that sparring for a beginner to advanced level practitioner provides many many more benefits then to a seasoned Master.