I don't see anything interesting or skillful in having an extra button to push. Your argument would have worked with vanilla WoW warriors because they had actual utility locked behind stances (our interrupt could only be used in Berserker Stance, our fear break/immunity could only be used in Berserker Stance, our stun could only be used in Defensive Stance), and even then it was clunky design that limited gameplay in certain situations (fear break macros were very unreliable, for example).
Also consider that forcing PLD, DRK and WAR to full-time Shield Oath/Grit/Defiance while main tanking helps keep MT DPS on a leash. It'd also be a step in relieving tank DPS from the duty of carrying groups to clears for DPS checks (since that's supposed to be the DPS' responsibility, not the tank's). You'd also be eliminating a factor to balance encounters around since it'd create a specific margin of expected damage from the main tank. Quite a few positives just from nipping that problem in the bud.
And as I've repeatedly said, this shit rolls downhill. It won't surprise me to eventually see people in DF runs giving tanks crap for sticking to their tank stances instead of switching like the "pros" do. This is the same playerbase that fostered WARs in 2.0 that would tell me to MT because "lol war cant tank" HM Garuda.It also isn't required unless you're in savage.