No extra damage resistance, but extra hit boxes still show a not inconsiderable boost in toughness. So someone at body 5 who takes 11P damage is down. Someone with body 1 and 3 cyberlimbs who takes 11P damage is still standing, even if it is quite hurt and taking massive penalties. Each cyberlimb provides an additional box no matter how many boxes you already have, or how much body you have. This shows that cyberlimbs are pretty durable, and provide additional durability when added to a metahuman body.
Reliability is nice also. No matter how many extra dice a limb gave, it could not resist something, but that extra box is always working.