I battle my Jacksons once in a while...closely monitored though, so it's mostly just a shoving match. Check it out:
http://www.youtube.com/watch?v=spYJvPCI1Vw
Alright, I just watched the video and thought I would jump in. I think this is a very interesting topic and definitely seems to push the envelope a bit.
ChamBust, when I first read your comment, I was a bit taken by it. My first thought was to the effect, "someone needs to call animal services on this guy, this is about 1 step removed from cock-fighting." I held off on commenting as I wanted to see how the conversation progressed. I read your justification and realized that there definitely seems to be some plausibility to your argument. I am not saying I agree or disagree with you, but I think you do have valid a point.
I do have some counters to your argument, however:
1) Just because something is their "natural behavior" does not mean it is beneficial to their health. Afterall, in nature many species do things like eating their young when food is short. Off the top of my head I cannot think of other examples but they do exist. Examples of behaviors which are necessary for survival or simply occur naturally, yet things I would not necessarily allow said species members to do in captivity. Human beings are a great example. In our natural habitat, we build bombs and blow each other up from time to time. If a superior intelligent being were to capture us and make pets out of us would it be a good idea for them to hand us nuclear warheads and tell us to go relieve some stress? I do not think that would be a good idea. (and before anyone objects, no I am not comparing humans and chameleons. I am attacking the argument that this activity is acceptable on the grounds that their behavior is natural by producing another example of natural behavior which would not be a good idea to encourage). Heck, sibbling get into fights from time to time, does that mean it is a good idea for spouses to have a fist fight from time to time?
2) The next question becomes- Given that this is a natural behavior in nature, is it actually beneficial to their health? Does it potentially harm them? That is, does it actually relieve stress? Does it cause stress? Will it extend their lives? Make them happier? I will not speculate on this, but will pose the question- Do you have any evidence that this activity is beneficial to them at all other than the fact that we see lizards fighting from time to time on Animal Planet? The heart of this is that just because something is natural, does not mean it is appropriate in captivity. Fighting over territory might play a very important role in nature for them as (I speculate) it encourages stronger members of the species to reproduce more by rewarding them with larger territories. This clearly does not apply in captivity (not normally at least), and I personally do not see any reason to assume there is benefit outside of evolutionary reasons (which are of benefit to the species as a whole, not necessarily to the particular individuals anyway).
There is also the question of what kind of tension/stress would this be relieving. Chameleons are not typically social animals so it does not seem clear to me that allowing them to engage in a dominance battle really does them any good. In particular, hypothetically, if one always wins and the other always loses- then it is just monitored bullying- how would any real benefit be assumed here?
3) "My two boys, Jax and Greemo having a highly supervised territorial shoving match (I separate them if they start biting). I've had them for almost a year and do this once in a while just to let 'em get out of their cage a bit"
This is the message that was posted under the video when I watched it. The way you stated "highly supervised territorial shoving match..." is very good. You have suggested that (from your perspective) this is meant to be a beneficial thing for their health and is not just a bunch of drunk frat boys who are bored lettin their lizards go at it.
What I would like to point out is that not everyone who has chameleons is mature about it. By posting a video like this online, you have portrayed fighting between chameleons in a positive light. Given where you stand on this, that seems to be a reasonable thing to do if you want to support your own cause. What I would caution is that not everyone who sees this video is going to think about it the same way you do. The question becomes- What role does this video end up playing in the promotion of responsible chameleon husbandry? Does it get across effectively and clearly the message you want it to?
Like I said, you have some very interesting ideas, I am not sure I buy them but there does seem to be some plausibility. I would encourage you to reexamine why you are doing this and whether it really is in fact beneficial to their health, and whether the risk of injury is really worth it.
[Edit]
"Like I said, you have some very interesting ideas, I am not sure I buy them..."
What I meant to say was: "Like I said, you have some very interesting ideas, I am not sure I buy them without solid evidence that it actually benefits them in some way and does not pose serious risk to their health..."
If your going for natural.
closely monitored 
c'mon...
