The beauty of the 6PPC is long barrel and case life. Many 6 ppc barrels can have a accuracy life of 3500 to 4000 rounds, with that said many times the brass will last almost as long as the barrel, you will probley go through a few hundred cases in the life of a barrel. Shooters I know that are ultra competitive, usually pull barrels at around 1000 rounds, and scrap brass at the end of 4 aggregates. They start with 20-25 pieces of brass, and if they shoot the same gun in light and heavy, and at both 100 and 200 yards, they will have 100 rounds for record. They will not shot the brass in competition until it has had 2 firings on it (Fireforming and one other firing.) then you have pratice rounds and sighters, so they may have 5 to 10 firings on a case before they throw them away.
I am not as blessed as them, and find this stuff hard to obtain, so I hold onto my cases much longer. I usually shoot my cases until they develop a "click" in them. (When you got to extract the case and at the top of the bolt handle at primary extraction, it takes extra force to open the bolt, this is a "Click.") I just don't like the hard extraction, so the case are no good to me. I shoot new brass with a new barrel, and heck I have some cases that have problem had 25-30 loadings on them, and have no click. The funny thing is you say that you have gotten an split neck on a PPC, I can not say I have ever had one. The important thing is having a die that closely match's the chamber of the gun, and properly having it set up. You also have to remember as the brass gets older you will have to re-adjust your dies a bit to bring things back into speck. This is the nice thing about shims.
I have a good friend that is a super competitive shooter are what he does is get the cases our one buddy discards after shooting 4 aggregates, and he will shoot them for a long time after that. He dosen't;t care about the hard extraction, and his cases have a click in them, and they still shoot competitively. (And think they was fired out of a different chamber to boot.) I hate to think how many firings the brass he is shooting has on it. I cant say I have ever seen him have a split neck either. I did see another friend that the necks would pop off and stick in the chamber after multiple firings, but I also believe that he cut his necks too far down when he prepped his brass, but even with that he had a ton of firings on that brass.
With hat all said I have no problem shooting a club match with old brass, but if I shoot a big registered match I like to have something a bit fresher. I don't think it makes a difference, but when you shelling out over $100 bucks in match fees, lodging, gas, etc, why gamble with it.
