Brother MacLaren said:
Could I see the numbers you used to arrive at this conclusion? Spring-Attacking rogue vs. Spring-Attacking fighter? By my reckoning (25-pt buy, core only, no magic), they are back-and-forth on damage, generally very close over most levels, IF the rogue can Sneak Attack and IF the fighter uses PA so his attack bonus matches the rogue's. It would be a more complicated situation to compare them against a range of ACs (say, ECL+8 to ECL+20), but that might give a better answer than forcing the fighter to PA for the difference every time.
Lets do that.
10th level half-elf rogue. rapier, chain shirt, no magic. 13 str, 17 dex, 12 con. Dodge, Mobility, Spring Atk, Weapon Finesse.
AC: 17. Hp: 47. To Hit: +10, 1d6+1 +5d6 SA (assuming spring atk into a flank)
10th level half-elf fighter. rapier, chain shirt. 14 str, 16 dex, 14 con. Dodge, Mobility, Spring Atk, Wpn Focus, Wpn finesse, power atk, cleave, wpn spec, imp. wpn focus.
AC: 17. Hp: 79 To hit: +15, 1d6+5.
10th level half-elf fighter. Greatsword, plate mail. 16 str, 12 dex, 14 con. Power atk, cleave, grt cleave, wpn focus, wpn spec, imp wpn focus, imp sunder, iron will
AC; 19. Hp: 79. To hit: +15/+10, 2d6+6
Since they are underpowered without magic, lets let them fight a CR 7 monster. Here is an ogre, 4th level barbarian (CR 7, MM) AC 17, hp 111, +18 to hit, 2d8+16. (raging)
The rogue can hit him on a 7 or better (5 or better if he's going into a flank). His average damage is 22. (range: 7-37). He will get rocked on any roll the ogre makes, and take 25 hp damage (or two blows and he's down). The trick to his damage is that the ogre will probably focus on the guy whose setting the rogue up for flank.
The LA fighter hits on a 2 or better, and can power attack for 10 (his max) to hit on a 12 or better. Assuming his dice are lucky and he power atks max, he hits for 18 damage. (range 16-21). Thats higher low end damage, but lower on average and on max damage. He has the same chance of being hit as the rogue, and can survive three such blows before dropping.
The HA fighter also hits on a 2 or better with his primary and a 7 on his secondary. He's not so comfortable with a full PA, so he power attacks for 1/2 (+5) which yields him +10 due to 2handed style. He must roll a 7 and a 12. (equal to the rogue on first, LA on second). If he lands both blows (and let say he does) he does 46 damage per round. (or between 8-56 depending on 1 or 2 hits and dice roll). His AC is only slightly better than the LA or rogue (2 points, ogre misses on 1-2) and he can take the same amount of hits as the LA (three).
Assuming the rogue's flanker misses every round (and thus adds nothing to the math) the rogue will down the ogre in @ 5 rounds. The LA will do it @ round 6, and the HA will do it in @ 3 rounds.
Now, there are two caveats to this math.
1.) The rogue MUST have a partner to do this. Otherwise, his damage drops to only 4 per round average. Similarly, the foes must be crit-able (or otherwise not immune to SA). If he has high-enough bluff, he can try to feint every other round (making the ogre ff) and this his damage becomes 22 every 2 rounds, or 11/round (and he will kill the ogre on round 10 exactly).
2.) No where in the combat does anyone hit, or confirm a critical. A crit does little for the rogue (only adding an average of +4 to damage) but greatly helps the LA (+18) and the HA (+21). This is due to the fact extra dice are never doubled, but extra bonuses are, so the PA does double duty, but the SA is static.
So, the LA beats the rogue primarily if he is alone, facing a non-SA foe, or facing a critable foe and critting alot. He cannot come close to the HA fighter standing there tanking, and the rogue beats his average damage output in typical party scenarios.
Therefore, I'm going to conclude that barring those corner-case scenarios (alone, no-SA, or a lot of crits) the rogue does slightly better damage than a full PA Lightly armored fighter of the same level. Neither hold a candle to the typical plate-tank fighter. So I'm forced to conclude the LA is neither as useful as the rogue nor the HA in typical combat scenarios under the guidelines given.