The cap on raid numbers is something of an established tradition in MMORPGs. The original reasoning was simple: if you don't have it, what's to stop a hundred idiots from simply rushing in and zerging everything?
I've spent quite a while in Rifts recently, where that's more or less exactly what happens (at low levels anyway).
Twenty DPS, one healer (me) and no tanks? No problemo! Just add more DPS and zerg that bad-boy down!
The thing is, it's all rather fun. Keeping a rampaging mob of half-wits alive is way more challenging than healing that I-have-a-horrible-feeling-you-could-solo-this-place-and-are-just-bringing-us-along-to-be-kind tank you've played with for years. Which made me wonder ... would it be so bad if raids were like this?
On the plus side, it truly would allow anyone who wanted to to "see the content". What's more, it would remove the non-linearity in difficulty that plagues many games. WOW, for example, only has two difficulty levels for raids: normal and heroic. If you're slightly below the ability needed to pass that threshold (e.g. your guild is good enough to kill Arthas with 11 people, but not 10), you get nothing. By removing the raid cap, that problem automatically goes away.
Of course it might be argued that this is unfair on players who are good enough to do the instances. Why should lesser mortals be allowed the purple shinies that were intended for the elite? Well, that's only relevant if you if you get your kicks from feeling superior to other people rather than the challenge of the encounter. It's not like the game is going to run out of epics if all the peasants get them. Besides, the skilled players would still gear up faster, because the loot is split fewer ways. If it takes fifty of you to drop a raid boss, you aren't going to see much gear at the end of it. Essentially, the system is self-limiting. There's no point in a skilled guild zerging, because the raid-lockout remains, so you still get your weekly kill, but the rewards are spread thinner.
Perhaps it's time to remove the cap.