You know, this has been debated pretty much since the inception of the (then) KCC on through the current version of the ACC. Personally, I think having cortosis is absolute crap.
1) "It makes things fair between a DJM and anyone below DJK." Well, technically, nothing should be fair between a DJM and anyone. If you have an advantage, you're going to press it to the full advantage. Also, if you're below DJM and choosing to fight a DJM, it's because you know it's not fair, and you either want to test yourself, or earn the DJM's favor, or whatever else. In any event, using cortosis to "make things fair" doesn't even do that. Lightsabers cut through pretty much anything; live with it. It's possible not to have your weapon destroyed; don't parry with your weapon, learn to dodge and duck more, and call foul play if you make that habit, and someone else writes you out of form, and destroys your weapons. Just an example.
2) "It's rare." Yeah, duh. It's rare. Yes, there may be ways to get it, but the fact is, you have to have the contacts, the cash/resources to purchase it with, and honestly, not "everyone" has that kind of access. It's restricted to enforce that, and if it were me, I'd probably deny it outright except as a prize for winning some kind of ACC competition, or Order War, or whatever. That way, you know what you have to do to get it, and it makes it extremely difficult for people to get it. Want it? Work for it, and be the best, show your skills off, and be recognized for it.
3) "What other alternatives are there?" Well, for one, there are "lightsaber-resistant" weapons. Sithswords are the most well-known, but there are other weapons that are created, or imbued with the Dark Side that have the ability to withstand the impact of a lightsaber. If you fancy that, I would be asking the CM for opportunities to earn/use one of these, or perhaps even talk to, say the Dark Council, about why we don't have competitions to earn stuff like this for official possession stuff, and whatnot, and when will they do something like this? (FYI, I actually would like to do something like this, which is why I mention it; stay tuned).
Another option is, "Hey, so-and-so. I challenge you to a duel. Yes, I could challenge you to open combat, anything goes, but I'd like to see how you fight on my standards. We'll fight like [name your terms here]. Do you accept, or do you deny it like a coward [or some other suitable insult]?" To which a suitably evil answer would be, "Worm, I defy your very presence, and I will kill you where you stand! [Force Lightning, dead junior, fun evil stuff." See, wouldn't that be fun?