Similarly, for those of you who noticed the spamming in your games, what do you think were the reasons why the PCs kept using the same attacks?
The reason we saw it happening is because everyone who tried to vary their attacks soon realized that it was suboptimal.
For those who don't like detailed math, see the handy table at the end.
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Say, for example that you are a 20 strength half-orc barbarian who is raging with a +1 weapon and weapon focus at 2nd level. That isn't completely out of the ordinary for our group. You are up against a Hobgoblin, CR 2. You have +11 to hit and you do 2d6+11 damage with your attack(with a greatsword). They have 16 hitpoints, so you kill them 75% of the time you hit them and you hit 75% of the time. I believe this gives you a 56% chance to kill the bugbear each round you attack it(although I'm a little unsure of the math).
If you decide to grapple them and you have an AC of 16, then the grapple is immediately stopped by the AOO 50% of the time, resulting in you taking extra damage and negating your entire action for the round. If the bugbear misses then it is an opposed roll, your +9 vs his +4. I'm really not good at the math behind the probability of opposed rolls. But my best guess is that in the 400 possible combinations of rolls between 2 d20s, you lose 117 times. That means you succeed 71% of the time. You then do 1d4+7 damage to the bugbear, not enough to kill it. This means you only succeed on the grapple 36% of the time and when you do, you don't kill it, you simply give all of your allies +1 to hit it.
If you decide to disarm it then you are looking at the same 50% chance of immediate failure and extra damage. If the bugbear misses, you are looking at +15 vs its +5. You succeed 84% of the time. This means that any round your disarm you will succeed 42% of the time.
If you trip him then you have the same chance of succeeding as the grapple attempt. Only you don't have the 50% chance of failure due to the AOO. You still have a 50% chance of taking damage however. You still need to make a touch attack, which you hit with 85% of the time. This means you have a 60% chance of tripping him each round.
As for sunder, it provokes the same 50% AOO. However, it succeeds 84% of the time. You need to do 30 damage to destroy the weapon(10 hardness, 20 hitpoints). On average you do 18. This means, it'll take 3 rounds to destroy the weapon if you hit every time.
Out of all these options, only the Full Attack has any chance whatsoever to kill the Bugbear in the round you hit it. Out of all the other options, only grapple deals damage at all. The rest of the options still require that you use some way of killing or incapacitating the enemy after you do them. Normally this involves a Full Attack and only ends up extending the combat another 1-3 rounds.
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Chance of Success(Half-Orc Barbarian vs Bugbear):
Full Attack: 75% (56% to kill the enemy in one hit)
Grapple*: 36% (80% with Improved Grapple)
Disarm*: 42% (95% with Improved Disarm)
Trip*: 60% (68% with Improved Trip, which means a 38% to kill in one round)
Sunder*: 84% (95% with Improved Sunder)
* provokes AOO(50% chance to take 1d8+2)
Most people in our group figured out the odds within a short while and realized that the difference between the options was so great as to be overwhelming in favor of using Full Attack every round. When your choice is to have a 36% chance of grappling an enemy(with a 50% chance of taking damage yourself) vs a 56% to kill the enemy outright, the choice is fairly clear. Especially when almost every other option is negated when the target has a second weapon.