I have been a guildmaster of a social-pve oriented guild for 4 years now, and during this time I have managed to do encounters such as sunwell, black temple, Ulduar, BoT etc.
As an organiser of raids and owner of a guild I can tell you, from a guilds perspective my opinion about this system
In an IDEAL world, this system could be used by guilds who lack 1-2 members before a raid. Instead of calling off the raid, you could use the raid finder, get some randoms in, give them your vent info, and at the end of the day those randoms could benefit from the loot of the boss, or even better, be added to some type of "friends list" so they can be invited for the next raid as well if they performed well.
How ever we all know that this isn't how it's going to work.
The 1-2 random people you will get, will be impossible to remove from your raid regardless of how bad they prove to be. You can (and will) end up with griefers who sign in as tank only to show up as dps, who are too impacient to wait for tactics and just charge in, who constantly ninja pulls to wipe your raid and so forth.
And notice how I am not talking about a BAD player who just stands in fire, but people who wilingly and purposefuly go out of their way to grief everebody else.
Now in a 5 man thats no problem, you can get people like that, it happens. But in a raid you end up ruining the mood for 24 / 9 other people.
Even further still, as more people will start raiding, they will say that the content is too chalanging and they can't do it. More "luck of the draw" buffs would be added, making it more favorable for loot horny people to join the raid finder rather then a guild.
So at this point I don't see the purpose for guilds anymore. Why not just remove the world in warcraft entirely and just give you a server list when you log on (like in team fortress) and you double click a server to join a pug instance.