OK, I guess I can see the need for something like a whiteboard for those that like tightly structured rules for things like effects and whatnot. Not my personal cup of tea, but now I think I understand why.
Even in finely-tuned-action-economy 4e, I and other DMs I know sometimes let players juggle order a bit. One such variant is to let PCs who roll the same initiative decide, round by round, which one goes first.
But what they can't do by RAW, much to my extreme annoyance, is act AT THE SAME TIME.That's something that any two PCs on adjacent turns can do anyway, simply by the first one holding his action for the second one.
As noted above - it would mean that sometimes folks could get a reaction that they'd not get if you were running it RAW.
It would mean that some spells could be made to persist just a touch longer than they would otherwise - a cleric could then choose to concentrate on the Bless spell until the end of the round before replacing it with another concentration-spell, for example.
I think the effect in 5e could be a bit profound. DPR:hp ratio is larger (or so it seems, especially as levels go up) in 5e. A hard encounter against a large number of monsters gets much harder if all the monsters get to go before any PC can act and much easier if all the PCs get to go before any monster can do anything.
I don't think it's game breaking by any means (even with individual init, it is still fairly common for one side to go entirely then the other)...it just means that playing smart, gaining/avoiding surprise and being in a good defensive position at the start of combat are all more important.
But what they can't do by RAW, much to my extreme annoyance, is act AT THE SAME TIME.
By strict RAW, during combat two characters cannot move together (example: one is carrying a silence device to cover the sound made by both as they move; or both are under an Invisibility Cloak).
Lanefan
And, obviously, the correct ruling.I ran into that issue last night. In the midst of a battle, I had a Nalfeshnee and a Glabrezu working together to pick up a stone altar that would have been difficult for either alone, and then they were going to carry it out the area of the silence spells that the PCs had been spamming to prevent them from completing a summoning ritual to open a gate to the Abyss.
I had one monster ready his action to help the other pick it up (though I suppose he wouldn't have had to ready, since RAW would allow him to help his ally with a check). Once they were carrying it between the two of them, I was going to let one creature ready his action to dash with the other character, and/or maybe even let him use his movement with it.
Technically, they couldn't even do the simultaneous dashing, but I felt that being able to carry an item together was a common sense enough capability to make a ruling allowing it.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.