Which is how it should be! AT-AT's should be so damn hard to kill that they wont need to RESPAWN because the rebel lines should already have been overrun (meaning the invasion of Echo base has begun and stormies could just spawn at a flag close to the base entrance or inside).

It should take a HELL of a lot more than 5 missiles or even 2 tow cables (since their is no way to trip up an AT-AT in BF2142 enginer) to kill an AT-AT.

Remember: the Rebels were desperately trying to stop the AT-ATs before they got close  enough to destroy the shield generator. The armor was too strong for blasters in this situation - if they relied on blasters only, there was no way they'd take 'em all down in time.

The whole point of withering fire is so that no one is completely useless. If you're a soldier with no more grenades and the AT-AT is walking by, you can hide or you can do your part, emptying your rifle and pistol into the thing's foot. You'll only take off 25 hit points from its pool of 20,000 (or whatever), but it might take an engineer another second to repair. One more second he can't be firing guns. Perhaps the extra second needed for the AV guns to take it down before it blows up the shield generator.

Withering fire is one of the very best parts of the mod, imho. In BF1942 on Guadalcanal where I was trying to defend a flag on my own - holding off soldiers and a tank (leaving me with very little ammo) only to have an enemy aircraft start strafing/bombing me... It was the best feeling to have damaged it with AA fire, and after he'd bombed the AA gun, to stand there with my pistol, shooting 'pointlessly' while he flies straight at me, guns ablaze... and seeing him suddenly burst into flames, unable to pull up, flying just over my head to crash into the hut behind me. 

Ever since then, I've been a big fan of the Withering Fire concept.  The idea that if somebody's vehicle is so damaged yet he won't stop to repair or get support to do it for him - he *deserves* to be taken down with small arms fire.   It's all about teamwork - keeping your vehicle functioning, or everybody firing together to chip away at armor that's "too strong".