You're right that this is the reason they are braindead easy, but I can say what I used to witness when they were more complicated:
- A PLD spamming the Rage of Halone (enmity combo) and never using Royal Authority or Goring Blade.
- As a PLD, I was once attacked for "not aiming my shield" at enemies which used to be required to block them (which was impossible because they were huge enemies in multiple directions that you couldn't move back to bring in front of you).
- A huge percentage of tanks using enmity stance 100% of the time even though a lot of raider tanks would use their damage stance almost 100% of the time.
- (In youtube videos and streams) I've always seen players not even pressing damage buffs like Fight or Flight.
- Obviously, to this day, they don't press damage reduction abilities sometimes.
- (In videos and streams) I've often seen people struggle to complete their 1-2-3 combo and instead do 1-2-1 because moving out of a red circle distracted them that much.
Yes, they do. It is all about how you word it. It is definitely harassment to try to tell another player what to do. You can make a polite suggestion such as "if you use total eclipse, you can make all of the enemies attack you in one hitGM's don't understand what is actually 'harassment'", but saying "do this, now" is an order and saying "you're bad for not doing that" is rude and is harassment.
It's also difficult to give polite advice in a dungeon because you don't know the person, so blurting out advice can be interpreted as "blurting out aggression" because they don't know you. It's easier to just give advice to people you know outside of the dungeon, such as FC members or players in the Novice Network, because they know you are trying to help since you are less of a stranger.
My mindset is that, yes, a player may not be doing well, but they are usually in an FC or the Novice Network and will, over time, ask questions in those locations and learn things from experience. I don't necessarily need to teach them it then and there, because in time they will learn it anyway and it's easy to carry them.