Let us not forget that Blizzard actually introduced the catch-up mechanic already in TBC. With the addition of Isle of Quel'Danas, Zul'Aman and the removal of needing to clear Tempest Keep and Serpentshrine Cavern in order to enter Black Temple and Mount Hyjal, they removed a lot of the gating that was between being decent geared and starting to raid end game content. It was NOT something unique for Wrath, even though it was more outspoken in this expansion.
People seem to have this romantic idea that Classic was such a good game, because only a select few got to wear full purple gear. I'd take a leap of faith and state, that the people who said that, are the ones who were in good raiding guilds, and got to clear content. For someone who was a good quality player, but started playing at a later point, it was absolute horrible. There was almost zero way of joining a guild as i.e a tank, since even with BIS preraid stuff, you would barely be able to offtank in molten core. Classic was a good game, because it was new and innovative in so many ways, but surely not because it was gated as hardcore as it was - and I did get to raid most endgame content, I still think it was a bad gamedesign though.
As Obnoxious is stating, the majority of people on the forums are hardcore players who has their stuff safe and sound. Of course they wouldn't mind an addition like that, since it wouldn't change much for them. But for average Joe, who may have limited time for raiding it will be a disaster. Not to mention, that while the population is healthy, there's just too few active, raiding, skilled people to implent what you're suggesting. Another thing to note, is that Ulduar is NOT something you clear, unless you know what you're doing. So what would happen would be a bunch of people, who for various reasons isn't good enough to clear Ulduar, will become stuck at just clearing Naxx, and not being able to progress from there. Fine for the hardcore players, but for the average guy it'll severely limit his options on the server.
EDIT: And finally it would make levelling alts to help the guild out in case they need to make changes to a raid setup or roster a complete nightmare