I would say it's a gimmick. Use Torkoal as a physical wall like it's intended to be. Hypothetically, you wouldn't use Amnesia on Cloyster or Barrier on Blissey because that's not what they are intended for. In other words, use Torkoal how it's intended to be, and save the other role you want it to fulfill for some other Pokémon.Amnesia on Torkoal...Gimmicky or Worthwhile?
My specific Torkoal has the following nature and spread...
Nature: Relaxed
IVs: 31 / 31 / 31 / 31 / 31 / 0
EVs: 252 HP / 252 Def / 4 SpAtk
...At level 50, this Torkoal has stats of 177 / 105 / 211 / 106 / 90 / 22. One use of Amnesia would bring SpDef stat up to 180 (at level 50), with an already decent HP and phenominal Def stats.
That's just my opinion though, you could always do that if you wanted, but I would prefer a mixed wall for that purpose such as Registeel