A level 10 rogue with 2 short swords and a 14 STR does an average total of 59 points with his 3 shots.
1d8+5d8+2 averages 20 x 2 shots = 40 points of damage.
1d8+5d8+1 averages 19 x 1 shot = 19 points of damage.
Ouch!
This issue has been hanging around since 3.x was released and, after playing several rogues over the last several years, I think I can say that it is really not as bad as it seems on the surface. Yes, a dual wielding sneak attacker has awsome damage potential, but that potential is limited by two factors:
The thief has only 3/4 BAB and not a lot of extra feats with which to buy combat abilities. It's true that they will probably win initative and get to attack their opponents flat-footed, but after the first round they are probably going to have a hard time hitting the DV of any meele-oriented character who pays any attention to their defense at all. My experience with rogue-types is that combat looks a lot like this:
miss
miss
miss
EXPLODE FOR A FUCK-TON OF DAMAGE
miss
miss
Those itterative attacks especially, are probably not going to hit. In short, a thief's damage output is unreliable at best. And if you are going to step into the middle of meele combat reliable damage output is essential. Which brings me to my next point.
The thief has average DV, little/no armor (DR) and lousy hit points (compared to meele classes). This translates directly into low combat endurance. And if a dual-wielding thief intends to stand still and trade full round attacks with a meele monster, low combat endurance is something he cannot afford to have. Again, it is my experience that if a rouge-type goes for a full round of sneak attacks it is because he is betting that the critter will drop before the end of the round. Because if it doesn't that means that on its next turn it is going to turn around and rip the thief a new one. Even a d8 hit die is not going to be enough for a theif to stand up for a couple of full round attacks from a power attacking barbarian or a meele monster (imagine a thief getting grappled by a Man-Ape :shock: ) There is a reason why the spring-attack rogue and the "feint/throw a dagger at you rogue" are both popular builds. Its called survival instinct.
What this boils down to is that sneak attack allows a rogue/theif to really bring on some pain .... once in a while. Which is how it should be IMHO. The thief who shoots for the brass ring in every combat is probably not going to live that long. Now, I will admit that the lowered Massive Damage threshold throws a curveball at this equation, but I don't imagine that a thief will prompt many more massive damage saves than a two-handed power attacker (let alone a two-handed power attacker with Wep Spec and Imp Crit! :twisted: ) If you find that too many people are falling to massive damage then consider using an easier save. I am considering giving 15 + (1/2 damage over 20) a try. But that is another thread. At any rate, my advice is to leave sneak attack and dual wielding alone, the damage will work itself out over the long run.