I think any raid caps are a poor choice. Design targets around a certain amount of people, and more importantly, design loot drops to supply that many people. If raids want to bring more, that's fine, but they get less loot per person--and since loot is rare, that loss should be a significant sting. But that's the trade off for lowering difficulty of the encounter--and that's what zerging does, lower the difficulty but without the obtuse need to recolor loot and change the encounter to retard status. (Instead, you just get less of it.)
The main reason I like systems like this though, is that they put more emphasis on strategic or big picture play. In other words, the difficulty usually falls on guild management, not on individual players. Guild officers need to decide what's a good "difficult vs size" for their players skill. Guild officers/leaders need to decide how they should recruit different classes to fit into their strategy. And the entire meta game should be set up to make those choices meaningful. A raid with groups composed or Rogues/Paladins/Chanters/Bards (Ect) should play completely differently from a raid composed of other class--buffs interacting should change based on the chains they form in groups. Forming a raid should be like forming a deck in a TCG card game. Recruiting players should be like searching for a special card to make your strategy work.
Why? Because again, all this difficulty falls on guild leadership, and people who are naturally more prone to devote more time and have a better understanding of the game. It increases the difficulty of the whole game, without putting the brunt of it on the individual difficulty the players experience. So the pot head who can come twice a week? Well, that's fine, because we don't have caps, and the management has set the raid up in such a way that he can only be a help. And he's not a hindrance if he misses certain encounters, because we didn't have to be so stringent on recruiting, so I can have two pot heads to fill one slot...Why? Because what's important is the LEADERSHIP and the "core group" of players knows the strategy, not the more casual person on the ground (They just need to know their job).
MMO's lost a lot of this "big picture" difficulty when they whittled everything down to pattern replication. And that felt like a bad move, because it pushed the difficulty on the lowest common denominator of every guild--how sane is it to simply expect the soccer mom who plays twice a week will be able to memorize an encounter if she can't make every raid? It's not sane, at all, especially when you have to shove all the difficulty into pattern recreation and you end up with 300 variable fights that would tax even skilled players to memorize. When that happens, what it ends up requiring that you have a difficulty shift downward to accommodate her, which, if your guild is trying a higher difficulty, means she can't come and has to find a guild which does the lower difficulty and eventually it means you need a "dumb switch", ala WOW LFR.
But if these decisions were left up to the players? Then management in a guild could build their strategies around perhaps having people like that. She could be an addition of DPS that hangs in the back, or someone that's brought to finish a buff chain, that will greatly help the more skilled players, without requiring her to do much. I could instead use the higher attendence players to do the heavy lifting. Since encounters are only designed with a set amount in mind--the "extra" or "support" players, like the soccer mom/pot head would be there to pad strategies, not key players in and of themselves--but the important part would be that they would be with their friends, DESPITE the difference in skill level. They would be defeating encounters WITHOUT the need to move to a different subset of players.
Anyway, long and short is that guilds should be able to design the way they play in the world, as much as the designers do. We've kind of lost that grand/strategic view of the world. It was another causality of the "lobby-->Game Room" evolution. And I'm not saying that evolution was bad. I, unlike most people, think WoW is a great game. But I think it would be neat if someone really explored the other direction--went on the road that was not traveled. Really explored making the "world" better, instead of the "game" (I don't think anyone is going to make a better game than Blizzard--but a more interesting world? Maybe.)