Its a solid question. Blizzard doesn't want it because they think the past is the past and for last tiers, it is the past. However, it is interesting to really think about how they can deal with the problem of players having too little to do when it comes to raiding. Why raid tier 14 when we can raid t16. Why keep seeing this raid over and over again? I am tired of this place.
There is no doubt in my mind that Blizzard has done an incredible job at designing their content. Not to mention, they may have just created the best raiding system out there. Flexible Technology being used on both Normal and Heroic Modes, and the 20 Man Mythic Mode. With this system, it makes me question on whether or not we truly need a tier skip system in WoD. Think about it.
Depending on how they choose to make their ilevels for LFR/Normal/Heroic/Mythic, they could make a system where no tier skips is even needed. Depending on how this works, the idea of guilds poaching other guilds for players may not even occur. Since tanks don't require defense cap requirements, or hit and expertise requirements are no longer needed either, there is very little to worry about when it comes to raiding or meeting specific requirements for specific bosses (mostly mythic though). It will mostly be specific class requirements (which has been a problem since launch).
I don't know if anyone here agrees but I do think that allowing tier skips to occur isn't the best way for players to move forward. Especially an old veteran player like myself who started playing again after 2 years since Firelands launch and am kinda disappointed that I can't do content like ToT or the t14 raids without feeling some sort of actual character progression. Just a thought.
Posted this on WoW Forums by the way.