It's all about deployment and strategy.
A squad of three war-walkers with 6 starcannons is death incarnate against a great deal of foes. But, there are situations you need to be INCREDIBLY careful with them, elsewise they're a waste of 300 points.
The difficult thing about using a squad of three is that getting line of sight with all three while remaining in cover is hard to accomplish. This is where risk assessment comes into play. If you wanna open fire, but have to expose yourself to do it, choose the target that you will most likely destroy to fire at. This reduces the odds of counter-fire.
Deployment and manouvering is key. Now, for a scout vehicle, the War Walker isn't very mobile. I don't see why they didn't just up the armour a little and up it's points too, but, then again I guess that wouldn't be very eldarish. Anyway, never move the walkers into LOS with an enemy unless you are planning to fire. Dodge between terrain obstacles, and use other units to cover them. Make those Walkers too risky to attack - that is probably the best way to counter their weak armour. If you've got heavy fire units covering them, the enemy will hesitate to move into attack position, knowing that a counter-attack unit is just daring them to do so. Also, distractions can do nicely, too. Rush that Falcon farther up the same flank your walker is moving, and cut loose with it's whole payload at something it can nicely destroy, or your enemy dosn't want you to destroy. That'll make all but the most seasoned veteran forget about those Walkers in order to deal with the more immediate threat you present them. You've got to play your opponent as much as you play your army.
Another thing to keep in mind with walkers is that sometimes, it's just not a good idea to use them against some squads. A squad of three walkers with six brightlances isn't going to do much against that mob of 30+ Orks -- though, those attack bikes over there are looking rather tasty. You just have to prioritize your targets, so that you take advanatage of your strengths.
Yeah, the walkers are kinda weak. And, yeah, after releasing their first volley, they probably won't survive more than two more turns. But, 18 Starcannon shots will most likely cut those Grey Knight Termies down to size, or shred that Tactical squad to ribbons. Put your shots into something that it can kill, and make them worth their points.
If you just manouver right, and have your units WORK TOGETHER to support each other, and prioritize your targets, War Walkers are one of the most bad-assed units the Eldar army has to offer.
