Raid teams tend to be hosted by guilds, and players who wish to raid tend to gravitate toward guilds that provide raiding opportunities.
One would be denying the most basic aspect of the raiding community in Warcraft to disagree with the above. I welcome any proof to the contrary. Please, prove me wrong. Seriously.
Given that assumption, I propose two more principles that compete with it:
The Guild is not the Raid and the Raid is not the Guild.
Guild membership is a player choice, made in concert with other priorities.
I will expound on each of these, to be clear on what I mean.
The Guild is not the Raid
Warcraft guilds unite around a common set of acceptable behavior. That behavior varies, so you have some guilds advertising "Be mature" and others saying "Anything goes". Level and gear doesn't matter, so long as you don't break the rules. This is a natural part of gameplay, and it allows the game to cater to us all in our own way.
However, when a raid team starts to form, additional rules begin to apply. Gear is checked for readiness, and performance is evaluated. Availability and attendance is taken into account. Stricter requirements form.
The Raid is not the Guild
Players that do not fit the guild's rules may sometimes be allowed in the raid. For example, foul language may be banned in guild chat. If a raid member does not use foul language in guild chat, but does in raid chat, or over Vent, a double standard is apparent.
This apparent hypocrisy brings to light the difference between the Raid and the Guild. They are two social circles that tend to overlap, but both have separate requirements.
Guild Membership is a Player Choice
No one else can tell a player what to do with the time they pay for. "It's my $15 a month" is the phrase that best represents the attitude I'm alluding to. Pressuring another player to change allegiance based on a single aspect of gameplay, be it raiding, PvP, whatever, is ineffective in the long run, and does not instill true loyalty.
Made in Concert With Other Priorities
Players do tend to flock towards guilds that provide the gaming opportunities that are in their interest. However, those interests may be in conflict. The stereotypical "No-lifer" with full availability, who can devote 40+ hours a week toward any given goal is just not common enough to be a realistic stereotype. Players are also people, with real lives, often a job, relationships, even other hobbies.
The most hardcore of guilds take these factors into account, and casual guilds tend to cater to such factors. In the middle seem to be a group of people who aspire to be "hardcore" yet they fail to recognize these factors. With such "hardcore" players, such things as having a job, a significant other, child, or pet, another hobby, or even a properly functioning digestive tract that needs occasional attention brands a player as "casual." Loremaster is "casual." Exploring is "casual." Fishing is "casual." Gradually leveling an alt is "casual." Doing old content is "casual." Anything outside their specific, personal goal is "casual."
Different players have different goals, and they must be accounted for. If enough available player's goals are aligned with a given group's requirements, have fun storming the castle. If there are not enough available players meeting such requirements, perhaps the requirements should change.