That's oversimplified. It will not be taken out, it'll just be disabled. Same will happen if you take down wheel of huge transport vehicle. And small walkers could also have good use. And it'll rotate better that any wheeled vehicle of same size. And btw, it's possible it could walk even with 5 legs if necessary (and more than 6 legs is OK too).But it's fantasy. Nobody is going to build a huge, barely rotating, slowass platform (especially a 6 legged one) which will cost tens of millions per piece which can easily be taken out by a pair of $80k rockets.
But I admit that walker in last CoD pic looks lame.
Btw, war sucks. Everything can be taken out by few nukes. Transport walkers could be used during peace too.