it's giving everyone involved the time to call defenses and make sure they've got a (more or less) accurate armor/body total in their heads during combat.
While I can't speak for everyone's experience, this is something I actively try to do, many players in OR and SEA do this as well, and we try to encourage this type of behavior as well; it makes the game more fun for everyone. If I know someone is knew, I'll try to avoid fighting against them and let other, newer, PC's deal with that NPC (PvP so rarely happens now these days, I feel it's a given that I'm not going to be fighting against a new(er) player as my PC unless they give me a reeeeeeeeeeeeeeeeeaaaaaaaaaaaaaaaaaaaaallllllllllllllllllllyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy good in-game reason for doing so). I just don't think we need a rule to mandate it, ya know? It should just be a cultural thing that players, ownership, and marshals all encourage and foster. I feel like the "be a good sport" rule kinda already covers this to some degree.
I also already slow stuff down for larger fights and battles, and I actively stay out of fights that have 4-5 people in them already (I mostly stand and watch to make sure everyone comes out alive/doesn't need help).
But those big bad guys, that are meant for my and my general character level (over level 40), I go all-in (add to that it's usually played by an experienced NPC/Player/Plot, or multiple people, or has a Monster Marshal).
My big concern, I suppose, is with larger field battles, with lots of Skirmishing and running around (we don't fight in lines out here in the West; we flank, we skirmish, we ambush, we run, re-group, look for our spots/moments). I feel like this punishes that play style, and certainly Rogues/Adepts since that is their primary method of fighting (if not in practice, then certainly in theory).
Look, if I'm sneaking up on a bad guy, I'm going to assume a couple of things:
1) It has a Magic Armor up.
2) It has at LEAST 1-2 defenses against what I'm going to throw at it.
3) I need to pop the Pre-Casts
---This can take 2-9 attacks, depending on the level of bad guy (you can Parry/Riposte normal attacks until you hear something you want or Magic Armor for, like Waylay)
4) Drop them as soon as I can because I'm behind enemy lines and need to use my surprise to my advantage. This means attacking quickly, not giving the enemy a chance to stop and collect themselves.
5) Then I have to "Nope!" right on out of there.
So if someone isn't ready to fight, that whole element of surprise is useless and wasted because I, out-of-game, have to make sure the NPC is ready and can react to the skills/abilities I am about to use on them, negating the whole premise of my planned combat action.
Fighting in a line just isn't fun for me. I've tried it in this game. And as a Fighter with ZERO magic items, I learned to skirmish instead. I even stopped using a shield about 9 years ago so I wouldn't be told to "stand on the line" against a bunch of casters throwing spells I have no defenses for.
Skirmishing is how I still fight, even with the acquisition of magic items. Why stand there and let some spell caster wear my defenses down and drive me backward, when I can loop around, come up from behind, and ruin their day? When I have to use a defense or skill while I'm fighting in a line, I nearly always feel like I wasted it.
Fighters without Magic Items are garbage in a line setup. I've been there and done that. All you end up being is a liability. It's a bad feeling.