A CASE STUDY
Here's a simple hypothetical scenario:
There's a Guard guarding a door, and a Barbarian wants to get in the door. The Guard sees the Barbarian approach around a corner. The Guard has higher INT than the Barbarian, but the Barbarian is much stronger. They both have the same speed of 30 feet.
DM: You see a guard is 30 feet away from you. What do you do?
Barbarian player: I move up and try kill him with my axe!
DM: The guard moves back and shoots an arrow at you.
What happens if we use the combat rules with @
Hemlock's initiative system?
Result 1: If the guard wins initiative, he backs up, shoots an arrow at the barbarian, and then the barbarian moves and whiffs at empty air. This is good news for the guard. He dealt some damage and maintained his distance. Next round, assuming the same actions are declared, the barbarian has just as much chance of catching him, so the encounter is still interesting. (Note: if both characters had decided to Dash, the possibilities of next round would be very similar: either the guard maintains his lead, or the barbarian catches up. Like tennis, it's not over until it's over.)
Result 2: If the barbarian wins initiative, he runs up and can make his melee attack roll against the guard. Then, if the guard survives, he can move back, which would provoke an attack of opportunity from the barbarian. Not a good plan. So maybe the guard aborts his movement, and just takes the point-blank shot. The encounter devolves into melee combat. Next round, the guard will probably take out his sword and duke it out.
(let me know if I got any of that wrong)
VERDICT: Both of these results are interesting, and the die roll in between decision and result makes it exciting. For comparison, let's see how this would turn out with the default initiative rules:
DM: You see a guard is 30 feet away from you. Roll for initiative.
(rolls)
Result 1:
DM: The guard wins initiative.
Result 1a: The guard moves back and shoot an arrow. Then, the barbarian realizes he needs to move 60 feet to get in reach. So he Dashes toward the guard, ending the round in melee reach. The guard takes out his sword and attacks the barbarian. The encounter devolves into melee combat.
Result 1b: The guard considers Result 1a, and decides he doesn't want to fight the barbarian. He Dashes away. The barbarian Dashes after him. Using cyclic combat rounds, the barbarian can never catch up. This is a "broken" result. The combat rules cannot resolve this situation.
Result 2:
DM: The barbarian wins initiative. What do you do?
The barbarian moves 30 feet and attacks the guard. The guard takes out his sword and returns the attack. The encounter devolves into melee combat.
VERDICT: First of all, rolling initiative is a very clunky way to start an encounter. Just saying.
Anyway, the big difference is that each player has complete information when they make their decision. There's no excitement, because the player knows their move will be successful. Note that the guard never needs to shoot a point-blank arrow: he already knows he's in melee, so he draws a sword. Also, since the flow of combat is so predictable, there's more pressure on each player to make the right move on their turn (because, like in Chess, you can think multiple moves ahead).
There's something counter-intuitive here. Cyclic initiative
feels more "strategic" because you have to figure out which move is best. But the strategy is actually much less interesting, because you can make the best move every time. If you roll initiative after declaring moves, you have to leave something to luck. As any pro Magic player will tell you, a game with a lot of luck can be much more strategically interesting than Chess--and much more exciting!
In my opinion, the "concurrent initiative variant" seems more strategic, more fun, and leads to more interesting outcomes than the core combat rules.