Hawken said:
Not exactly. I think its a good rule. But no one so far has presented any compelling reasons against it. Much of these reasons against it seem to be premised on the idea that rogues should be able to do SA while being as good or better at combat than fighter classes and that there should be fewer guidelines on SA than there currently are.
Good reasons are often only accepted when you already partially agree with them.
Personally, as a player, I would be less inclined to play a rogue, particularly any rogue-like character who wanted to be a dual-wielding type, or a swift ranged type.
Now on to so specific comments:
Sneak Attack & Power Attack
Example: Rogue wields a longsword, his opponent is unaware of him, but he knows that his opponent is incredibly resilient. So he takes his chances, winding up his swing to slash his longsword down upon the back of the neck to hopefully take him out in one fell swoop. Rogue knows that he's sacrificing the finer points of anatomical precision but he still has the better knowledge of where to strike and how to work the blade once it connects.
Result: Rogue power attacks for X, taking a chance to accidentally miss, where he'd normally be pretty certain to hit, but figures the extra force he'd put behind the blow is worth it in comparison.
Logic: A rogue power attacking is actually trading away more than a normal character power attacking, often because with sneak attack he is going to deal more damage anyways and should focus more on connecting his hit since his sneak attack damage will then kick in. Decreasing his to hit (which is often lower than most power attackers, read fighters or barbarians) is more to his detriment unless he's fighting a considerably weaker opponent.
Sneak Attack & Combat Expertise
Example: Rogue is fighting a powerful warrior, so he's on the defensive, however Fighter moves in to flank the powerful warrior. Keeping his guard up, he lets Fighter take some of the power warriors attention away from him, jabbing in with a powerful stab here and there when he gets the chance.
Result: Rogue knows that if he lets himself get hit, he's dead quick, so he keeps up his defenses even though he's flanking and taking the chances to get in a sneaky blow here and there.
Logic: Again, the rogue taking a penalty to-hit is more of a detriment when he's in a sneak attack situation than the benefits taking the penalty to-hit provides for Combat Expertise provides. He's going to do considerably less damage taking that penalty to hit anyways, so when he gets that occasional hit and does some sneak attack damage, he's only making up for the majority of times he's missed when he might otherwise have hit due to his to-hit penalty.
Two-Weapon Fighting/Rapid Shot & Sneak Attack
I'll not do an example here, but when it comes to these feats, they have taken a feat to get the benefit to attack at an accelerated rate with a penalty. Inflicting additional penalties upon these characters is inflicting a penalty upon any character with these feats who can sneak attack that other characters don't have to deal with.
Overall Logic
My general thought is that you are inflicting penalties on something that already costs a character a significant amount (a feat choice). Which for rogues particularly is relatively limited anyways.
You are making rogues choose between interesting combat-options (two-weapon fighting, rapid shot, etc.) and the only damage dealing capacity they are given by their class. A trade-off which is less than fair to those characters.
And not many people agreed with your ideas, me among those disagreeing.