Why should grappling provoke an attack of opportunity? I'm not talking from a realism point of view, because yes obviously, an untrained person trying to grab a man with a sword is in trouble. (Though I've seen ARMA videos where folks with swords do a pretty good job of closing in for a grapple as part of their weapon-based attack.)
I'm talking about for a 'simplifying the rules' standpoint. Just assume that you know what you're doing, and require a touch attack and a grapple check. Adding in an extra attack roll just slows things down, and grappling is difficult enough to pull off already.
I'll cover this and mmadsen's comment about kung fu:
mmadsen said:
For a lot of kung-fu action, the grappling isn't really mechanically grappling at all. The last attack that finishes off a mook happens to throw him on his head, or into a table, or whatever
Simplifying unarmed combat = good. Amen. As a martial artist, I like to at least make sure your rationalizations are accurate. Basically, justify your rule with the right example, not an incorrect one.
I'm no karate super star. But I do hold a black belt in Ishen Ryu, and have had some training in grappling and weapons. I have some practical experience, as it were.
In karate, the hard styles (like Tae Kwon Do) are kicking and punching. Those pretty obviously translate to attack rolls for 1d8 damage when the Monk goes all kung fu on the bad guys. Nice and simple.
In the soft styles, like Aikido, Judo, Hop-kido, Ishen Ryu, and others, the combat is more akin to grappling. Or at least it can end that way. In my style, we do the usual kicks and punches, but we'll just as easily throw in aikido or hop-kido moves to sweep, throw, or trap our oponent.
In sword fighting, that too can quickly devolve from trading blows like Erol Flynn to punching with a pommel, trapping the enemy's weapon, and throwing them to the ground.
As my training covered, most fights (even bar fights) end up on the ground as the two combatants grapple, instead of trading blows.
So, Grappling ought to be easier and more common, not harder and less likely.
In real grappling, while I'm trying to do something to you, you're actively squirming out of it and trying to do something to me. Whereas in D&D, it's very much about taking turns. Presumably, AoO can sort of simulate this non-taking turns aspect, but it normally represents putting yourself at extra risk such that the other guy really has an extra chance of hitting you.
This latter, common aspect of AoO happening when you expose yourself is the opposite of grappling. Nobody in their right mind tries a grapple move in a real fight if the enemy was ready for it or if it exposed yourself greater than normal (technically, swinging your sword to hit the enemy, exposes you, so every attack should induce an AoO).
I'd therefore say that making a Grapple move (trip, throw, disarm, whatever) does not induce an AoO (as RW suggests). I'm going to sweep you because YOU are vulnerable to it, not because I like making myself a bigger target.
Additionally, a Grapple move may also be valid as a combat option if the enemy induces an AoO. You move stupid, I get a chance to Grapple you or swing my sword at you.
Now to cover mmadsen's comment about a take down being the end of a mook. While I can hope that me throwing you over my shoulder = you being KO'ed, I'm throwing you over my shoulder so you take damage from the fall. You may or may not stop fighting. Which is why I may just as soon hang onto your arm while you fly over so I can begin my next bit of damage infliction on your shoulder and elbow.
Assuming no other opponents to worry about, I can probably render you innefective by throwing you and manipulating your arm (and/or breaking it) as part of the throw such that I'm still on my feet and you are on the ground while I twist your arm into painful positions. If I let go, you may get back up and fight some more (depending on what I'm doing).
This is just a sense of what someone with unarmed combat skills can do. I'm OK if the rules don't fully model it or simplify it. It's nice when there's some alignment with reality and still keeping it simple. With most of the grappling rules, they always felt like they were designed by people without any fighting experience and thus made overly complicated.
It'd be nice for unarmed combat rules if:
weapon fights moved to punching, throwing, pinning as they progressed
grappling was as effective as attacking someone to death (I can sword you for six rounds or sword for 3 and grapple for 2 rounds)
grappling doesn't become the I Win button for strong characters in a fight (like the Chain Fighter of 3e)