First off, I am not in a guild that would be considered "top end" by any reasonable human being. WIth a wife, job, column, and family, I don't have the time it takes to play the high-end raiding game. I've been there and done that though, so I do believe that I have some perspective on this issue.
There is an increasing number (thankfully) of players out there who realize that playing an MMORPG is not about the "lewts". It is about experiencing the content. Most players don't have the time or inclination to put forth the effort it takes to be on the bleeding edge of content. That is no slight against those who do have the time or inclination, but if playing the end-game were easy, more folks would be doing it. When I was younger, and had more time on my hands, I could afford to play EverQuest (yeah, this is a while ago...) for 6-8 hours per day during the week and for marathon 12 hour sessions on weekends. 40 hours of gaming per week was considered "a good start" for many high end guilds back then, and that hasn't changed much today. Sure, a good guild can clear something like Karazhan in only a couple of hours, and many dungeons are even faster. However, it takes time to farm materials for consumables, farm components for tradeskill items, and farm dungeons to properly equip a guild for the next level. I will not argue against the idea that it takes skill to defeat some of the end game encounters in an MMO, but the far larger component of end-game success involves time. Once a boss is on "farm status", a guild still needs to kill the mob several (dozen) times to make sure everyone gets the gear they require in order to progress.
That is where "welfare epics" come into play. While understanding that the term "Welfare Epics" originally applied to PVP Arena rewards, the term is easily applicable here as well. Instead of farming tier 5 and tier 6 gear for an entire guild, you can leapfrog ahead by running heroic instances or mastering trade skills and have the gear required to experience tier 5 and tier 6 content without the massive (no pun intended) learning curve or gear farming experienced by most end game raiding guilds.
Some folks might think that it's making things easy for the casual player, and they would be absolutely correct. However, Blizzard appears to have learned a very valuable lesson. It is not cost-effective, in the long run, for them to design all of this great content that only 5-10% of the player base will ever see. When Burning Crusade went live, guilds that were more casual never gave Zul'Gurub, Ahn'Qiraj, or Naxxramas a second (or even first) glance. That is a lot of time and effort (read: product) from Blizzard that customers never saw, but paid for anyway. This time, while the end-game guilds have moved on to Sunwell Plateau (or even Northrend, when Wrath of the Lich King is released), the average casual player will have had a chance to experience more of Burning Crusade's content. This gives an expansion more value to the casual player and makes it more likely that the player will not leave WoW for any of the new games coming to market soon. Giving players more content to experience is a very profitable proposition for Blizzard, and making "not-quite-leading edge" content available to casual players requires no money or development time on their part. The top end guilds will move on to the newest content (Sunwell), and casual players will have plenty to do while waiting for Lich King to arrive.
So, to answer the question, if I were an end-game raider, I wouldn't be concerned that casual players got to experience Black Temple and Mount Hyjal. I'd be looking forward to Sunwell Plateau. Playing in a high-end guild means staying on the leading edge of content or as close to it as your guild's time and skill allow. Regardless of how many other guilds get to end game content eventually, I would focus on the fact that I got there first. If a player doesn't have the time or inclination to do what it takes to stay on the leading edge, then he should relax and take comfort in the fact that he will get there eventually. It's all about the pace at which you want to play.