I know it's kind of beside your main point but this is completely false. Like many, many guilds mine cleared all of MSV the first week it opened without a single piece of that rep gear, it was not a huge disadvantage by any means. It's just that gamers have an obsessive min/max mindset where a disadvantage of any size automatically equals a massive disadvantage. If it's possible to be 0.0001% better geared then it is ABSOLUTELY MANDATORY that you be.
(Actually this is a major issue with the concept of content gating and gear progression - a large part of it is due to the mindset of players. You didn't play 24/7 for months to improve your damage by 1%? OMG what a noob!!!)
A better example of gear-gated content in WoW would be say heroic modes which are tuned to require at least a few pieces of normal mode gear. Although world-first guilds down many if not all of these heroic modes in a week or two with very little of that gear, so for them at least that gate is not unpassable (little harder for us mere mortals :P). More accurate might be that beyond the first tier of an expansion subsequent tiers are tuned to the level of the previous one (roughly NM previous tier level?).