A lot of players (of all roles) don't really understand how threat works in this game, and I can't blame them because the in-game tooltip doesn't exactly make it crystal clear. The guard is almost useless on a healer (speaking of pve only ofc). But if you're not worried about losing aggro to either of the dps players, you can guard the healer by default.
And yes, you're right, it's probably not worth arguing about in the middle of the flashpoint. As a tank, you don't want to get on the bad side of your healer(s).
Now if the healer is drawing aggro, most likely it's because a mob somewhere has been left alone by everyone else. Whose fault it is exactly is debatable, but as a tank, you can't be everywhere (though you should strive to be !), and everyone can make efforts to make your job easier : dps should kill weak adds quickly and engage anyone you can't handle yourself, and a healer under fire should learn to los, stun, and stay close to you rather than running away or being a sitting duck.