In casual content, the second tank is very rarely more than a nerfed dps. Most HM primals, all 8 man dungeons you will just be dpsing. In 24 man content (minus CT), in most situations all 3 tanks have clearly defined targets, but on the boss fights you'll resort to being a dps. It happens.


But that's casual content.

In Midas (non-savage), you start to get taught about things like stacking debuffs (which force tank swaps), mob management (tanking mobs that spawn) and encounter mechanics (which may involve non-damage related mechanics, like leviathan HM/EX).

As we get into more difficult content (such as Ifrit Ex, Titan Ex, Garuda Ex and Sephirot Ex(for example)), we begin to see these mechanics become more central to tank gameplay.
More stacking debuffs (which necessitate a tank swap to survive), more adds (which the MT cannot tank due to the mobs providing buffs to the boss when close) or damage that needs to be split between tanks (i.e. Sephirot's 2 pillars). These give more focus to the mechanics that tanks will have to get used to, i.e. swapping to survive tank busters, managing adds/defensive cooldowns and dealing damage effectively.