I feel like this was an intentional design between WAR and PLD actually.

Brutal swing is off the global cooldown, but can't be spammed.
Spirits within is also off the global cooldown, but can't be spammed.

Because many bosses require either stun or silence, this makes both of the classes useful as an MT or OT depending upon the situation, or simply bringing the both of them.

Ifrit HM can be stunned. While it may be difficult to stun as PLD while running as the main tank in this fight, WAR has it a bit easier since brutal swing is off the global cooldown. On the flip side, a PLD can off-tank and only worry about stuns so that the MT can focus on keeping threat.

In ADS-type bosses, a silence is necessary. WAR doesn't have a silence ability, so it is useful to bring a PLD. Silence was not mentioned in the OP, but I think it is the balancing factor to consider.

I myself am pretty happy with this balance because it gives a reason to use one tank or another in each situation.