So I spend the next 2 years building up this awesome, working town of Elkagorasa'shire. With everything I need to level up my gear, start raiding and maybe even some world PVP. I am totally excited that I'll be able to guide my progression through the game and tailor my daily quests to what I love to do. Plant a farm, go fishing, make GOLD on the AH... all at my garrison. I can almost hear the jingle in my head.
But like any expansion, I foresee my Pandaren farm going fallow. Why would I need to plant Windwool cactus while living in Draenor? My garrison is already going to have a farm ran by an NPC. It's highly unlikely that I'll even visit our panda buddies after leaving the giant floating turtle island.
Zooming ahead 2017. Elk is sporting his LFR iLevel 703 gear, doing extremely awesome 2000 dps, and is packing his backpack (and possibly scuba gear) for that inevitable next expansion. Now the place Elkagorasa has called home, building it from the ground up, he is simply to give up on it? I don't think so.
Option 2: Take it with me. The Dalaran mages have already proven the technology exists. I'll simply have the goblins shrink it down, and hitch it up to the back of my Horde-a-cycle and bring it with me. When I get back to Orgrimmar, or wherever the dangers exist, I'll just pop the top, and blamo, I have my garrison.
Blizzard, if you want to keep old content viable and visited regularly, let's make a reason to go back there. I'd continue going to Pandaria, if my Tiller farm could grow alchemy mats for future expansions. Garrisons look to have a larger time commitment than the Tiller's rep grind. This is one item I don't think I'll want to give up easily. Blizzard, start thinking now about making it portable. The Dalaran mages did it, why can't I?