i was just wondering what happens when you have a squad of war walkers say 3 and one gets immobilized, do you
i) leave it behind like a casualty.
OR
ii) keep unit Coherency and not be able to move the rest of the squad for the rest of the game.

could anyone clear this up for me?

T.I.A.

Aheon.

I think you can do either last time I checked the rules, but if you're asking that as a tactics question, it is up to you. As Warwalkers don't rely on their movement as much as, say, Vypers might, there is no point in moving them if you have enemies in clear view, thus keep them in coherency with maximum fire power. If however you're facing faster foe and you fear you might lose line of sight (or don't have it in the first place anymore) it might be better to move the remaining two so they can keep firing. Better to lose one then waist the whole squad.

Someone might want to double check the rule book however, just to make sure you actually have the option. I don't have mine at hand.

Cal

As Calaith just stated you can do either. However if you choose to abandon it, the stunned or immobilized walker is destroyed. If you are talking about the tactical advantage, it is different depending on an individual situation. If you are facing a slow methodical army, shooting is more important than movement. But if you are facing a fast army that likes to play Hide and Seek, movement is more crucial than shooting.

The rules are on pg 73 in the Big Green Book.