Whoa, whoa, whoa...
Yes, two shortswords suck during a Spring Attack. Not denying that. Two shortswords suck during a charge attack. Not denying that. And yes, two shortswords suck if you are far away from the melee and do not have access to Haste.
Lord Zardoz---
When going for high damage, dual weilding two shortswords is not the greatest of attack options. Generally when dual weilding, it is better to go for something like 1 Long Sword + 1 Short Sword. Against large numbers of weak enemies, using the paired short swords is probably a good choice.
What SpikeyFreak said.
Lord Zardoz---
Also, if you had a single +2 Quarter staff, you could dual weild that, and get the benefit of 1.5 str on primary attack and .5 str on secondary attack instead of 1.0 and .5 with the two short swords.
Sorry bud. You get 1.0 Str on the primary attack on a quarterstaff if you wield it as a double weapon. You only get the 1.5 Str if you are wielding it as a THW (and not TWF).
SpikeyFreak---
When going for damage, greatsword is the way to go. Failing that, a double sword.
Granted. A two-bladed sword is better than two shortswords in all instances. (Except when grappled.)
But I disagree that a greatsword does more damage than TWF. You are neglecting a couple of points:
Weapon Specialization
Enhancement bonuses (GMW)
Let's go with 11th level: (The original poster must be at least that to have 2 +2 shortswords and a +3 greatsword.)
Assuming L11 Fighter (we could put in L1 Ranger, since that's how most people would get the TWF/Amb, but I'd like to keep this consistent).
Greatsword: 3 attacks
1 2d6 +1.5Str +en +WS (+11)
2 2d6 +1.5Str +en +WS (+6)
3 2d6 +1.5Str +en +WS (+1)
It is not likely that attack #3 will hit.
TWF: 5 attacks
1 1d6 +Str +en +WS (+9)
2 1d6 +Str +en +WS (+4)
3 1d6 +Str +en +WS (-1)
4 1d6 +.5Str +en +WS (+9)
5 1d6 +.5Str +en +WS (+4)
It is not likely that attack #3 will hit.
Now with GMW, the enhancement bonuses are the same. Also, I'm assuming that both of them have Imp.Critical and keen weapons. (The Greatsword user has more feats and more money to spend on his sword, besides the fact that he probably multiclassed as a barbarian. I'm going to disregard that for the moment. I'm also going to assume that you realize Power Attack is worthless on a full attack.)
So since attack #3 is unlikely to hit, let me take it out of the equation, to get: (Obviously, the #3 for a greatsword is better than for a short sword. I'll add that back in later. I'm just trying to look at most likely scenarios.)
Greatsword: 2 attacks
1 2d6 +1.5Str +en +WS (+11)
2 2d6 +1.5Str +en +WS (+6)
TWF: 4 attacks
1 1d6 +Str +en +WS (+9)
2 1d6 +Str +en +WS (+4)
4 1d6 +.5Str +en +WS (+9)
5 1d6 +.5Str +en +WS (+4)
Assuming that all attacks hit (which is a big assumption, I know), then:
Greatsword = 14 +3Str +2en +4
2SSwords = 14 + 3Str +4en +8
(en = +3 for an 11th level GMW)
Greatsword = 24 +3Str
2SSwords = 34 +3Str
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Now, these are not the final numbers. The greatsword user will hit a little more often, tends to have a higher Str (by 4 points or so), an extra special ability on the weapon (due to having only one weapon to enchant) and the 3rd attack will do more damage overall. However, this is offset by the fact that the shortsword user crits almost twice as often (due to more attacks) and thus their WS/enhancement bonuses become even more crucial.
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In short, I have seen TWF played effectively. (With two rapiers, short swords, bastard swords and a two-bladed sword.) I have also seen THW used properly (keen falchions, scythes, [didn't have merc greatswords], greatswords, glaives, spiked chain).
The only times when I've seen TWF effective, however, is when the character doesn't max out their Dex (or go with Weapon Finesse [bleh!]), and instead maxes out their Str, as any good warrior does.
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Side note:
I think it's funny that Spring Attack/Mobility/Dodge are more important for a THW character than for a lightly armored TWF, since a TWF is trying to get as many full attacks as possible, whereas a THW character can use Power Attack/Spring Attack. Funny, no?
(Besides the fact that I think Dodge/Mobility/Spring Attack are worthless feats. I'd much rather have Iron Will, GF and LR. But that's just me.)