I've leveled four tanks. Two, warrior and druid, I leveled before heirlooms existed. The other two, death knight and paladin, I leveled using dps heirlooms, which were more than adequate for tanking while leveling. My warrior, which I leveled before heirlooms existed? Back at level 70, I tried to tank Utgarde Keep when it was released. I found it to be frustrating because the mobs weren't hitting me for any notable damage so I was rage starved completely. In response, I threw on my pvp gear to tank with and all problems dissipated. Because? Leveling dungeons don't want or need tank gear. All tank gear does is gimp you in those dungeons.
Why do these heirlooms exist? They aren't necessary at all. Maybe a shield and a weapon. Nothing else is even remotely wanted.