Zay's system seems overly-complicated, but then I'm sure that over-complication is more of a placating effect on the overall guild. Most people have seen/heard about larger guild simply imploding for one reason or another and there's a good chance it's loot-related or leadership-related. There's just way too many variables involved in trying to make everyone happy, and that's ultimately what's done. Trying to make it fair for everyone so everyone (in theory) is happy.
I can see how IRPP can be used to motivate people to participate in gathering supplies for raids, herbs for potions, DI ore for FR gear, farming all core of elements, skeletal fragments, etc. for Naxx gear, etc. but why would you need RaidRPP? If they earn and spend it why would you need a total tally of how much someone has earned in a lifetime? How does 3wap bear into the equation when loot drops? Is that simply there to assure people who raid more often get a better shot at stuff versus those who show up a once a week?
The system actually isnt too complicated really, maybe just because were used to it for the better part of a decade now. I think the key to a successful loot system doesnt lie in making the system "fair" numerically. It lies in how you view equipment. We've always looked at loot as a tool of progression rather than a shiny reward for your hard work as most people seem to see it. Most of our membership sees it that way so outside of a numerical system, they use common sense and say, "Soandso Rogue is still using a T1 weapon, let us(all the rogues) pass to him".
Thats not to say this common sense can backfire easily. For instance some heavily geared guys going into a new progression zone who havent upgraded in a long time and have had no need to since they still top the performance charts. Despite other people needing the equipment, and that overall it'll increase the entire raid's effectiveness, those people may feel like they want something new anyway. And thats where the numerical system comes into play I suppose, but then you risk ill will going the other way also with people feeling like that guy was greedy since he isnt helping the raid as a whole. Its a vicious cycle.
The people we recruit, we try our hardest to make sure they're people who fit into the mentality that equipment is a tool of progression, not just something to look cool in around IF. It also helps that we have an almost zero turnover rate on raiders, weve never really had people leave for greener pastures, maybe like 4 or 5 people in almost 8 years.
As for the other question, RaidRPP is a measure of how experienced or how much time someone has put in as a whole. We use it sparingly and often in non-gameplay situations. Such as when we got 25 TBC keys, we gave them to the top 25 RaidRPP. Another instance it was used was when we were determining who would start collecting shards for the first Atiesh, we went by functionality first which put it on a mage, then RaidRPP, no one objected openly or privately so it seemed to work ok.
3wap is sort of defunct now, in EQ we would use it as a small weighting factor on whether or not someone is eligible for looting an item. If they had a really low 3wap meaning they were probably gone for a couple of weeks, and their competition had acceptable LottoRPP and 3wap (not super negative, consistent raider), we might default to the other guy. We really dont do that anymore in WoW since equipment is much quicker and easier to obtain and not as diluted.