Raid Lockouts & Casual vs. Hardcore
Tobold recently discussed the effects of raid lockouts on casual guilds, so I'll cover the hardcore side.
One important aspect of raid lockouts what also ties into Tigole's mention of fostering competition between guilds: Raid lockouts act as a pacing mechanism for inter-guild competition. If you need a certain gear level for an upcoming raid encounter, you can't just raid the previous raid encounters nonstop and have your whole raid equipped to the brim. Since you get just a certain amount of items per week, you can't compensate for lack of skill with extra time spent to progress. A perfect example of this is the Chinese guild The Seven, who killed Illidan in just 52 days. An average guild might spend 52 days just farming Karazhan to get gear to kill Gruul, The Lurker Below and/or Void Reaver. If Blizzard wants to encourage inter-guild competition in PvE, they must also discourage long raids and other unhealthy playing habits to avoid bad press. And that probably means that raid lockouts are here to stay.