you know reading these comments makes me wonder WHY they don't add it because like you said...immersion. I think it makes roaming the wild and exploring and just being out and about more fun and more cautious. I miss the times when someone would accidently AOE a close mob and then everyone perks up and the battle plan has to change and teamwork is needed...maybe even a few fallen at the end of the fight but all this MAKES A BATTLE A BATTLE...isn't this what makes a battle system fun??? maybe this little things is what XIV battle system is missing.
Curious ones may follow but may be become frightened and run upon player action and certain ones will join a battle if still in range and certain ones that may only attack if a healing spell is used and attack the healer.
I think its little details like this that actual keep us on our toes and on the look out.