This is something, until now, I hadn't given too much thought. Although, it may be possible to climb your way through the ranks and earn a black belt in several systems, I seriously doubt anyone could do that with high ranks. To me, the ability one has in their core style is the real measure. So, I think there are legitimate reasons that someone might receive high ranks from several people/places.
Foremost, is it can represent that other masters or seniors of that art recognize your talent and knowledge. For example, it may be that one FMA teacher is so skilled that he has been recognized by several other varieties of FMAs. The same thing can happen in Jujitsu, Aikido, or Karate. If you trace Kenpo just to Chow, there are hundreds, if not thousands, of "Kenpo flavors." Conceptually, there is a lot of overlap. (Yes, I know YOUR Kenpo is different from the rest.) So, isn't it really that much of a stretch for someone to obtain a high rank in another and then be recognized by the cousins as a skilled-fellow Kenpoist?
So, in my mind if you have legitimately achieved a high rank (with real skill and knowledge) in one style it is possible your peers may recognize your skill by awarding you rank in their style. I think someone suggested that these were sort-of honorary degrees. I think that is a good analogy. It represents superior accomplishment. I don't think anyone that gives out these ranks really expects the recipient to start teaching that style. After all, if you are highly regarded in your own art why would you suddenly drop it to teach another? If you were legit, you wouldn't.
After you are recognized the question is what to do you with the recognition. If you are putting it on your letterhead or business cards, you're probably taking it too far. (I'm looking at you Stephan Colbert, D.F.A.) But if you are trying to entice students to you or make a little green, then putting on a website seems OK to me. Or even hanging the certificates around the school seems fine. The first-time-guy-off-the street may think, 'wow, I can learn all this here.' Although he's wrong, if the recognition is legitimate, no one is injured by his mistake. He got a good instructor that is well respected, sounds like a win.
Unless the person is a complete fraud, seeking certificate after certificate without any competence, I'm not sure why we care. There seemed to be some sense that if you where a rank you are somehow constrained to teach only the material that got you that rank and to do otherwise is disrespectful or fraudulent. First, this is based on the erroneous premise that all of Kenpo brand X is the same. And I don't see how knowledge can be so assigned. If I show someone how to pass a guard while wearing Kenpo patches and black belt, but don't have a black in Jujitsu, then I'm being dishonest?!? Does Jeff Speakman have to take off the rank Mr. Parker awarded him because he's doing Kenpo 5.0? Or how about Chuck Sullivan and his IKCA system? Can't showing somebody how you do the technique be enough for others judge if it's crap or not? Do you have to start every lesson or demo with a disclaimer?
I'm reminded of the saying "Just because the stripes show, doesn't mean you know."