I was underneath the car the other day and I noticed that both drive axle covers are very rusty.

 

that is just the outside of it. it will not rust apart or anything.

Mine looks the same. I expect my cv shafts to be replaced because of bad bearings FAR before that rust does anything.

 

Rust like that is not an issue, You'll notice a lot of rusty things down there, like suspension bits and etc, axles. But that's just surface rust and doesn't eat up the metal components.

Sheet metal is affected a lot worse from rust due to its thin characteristics, and rust is able to eat up these parts a lot quickly.

 

Well that maybe so but it does show you where Honda used cheap steel instead of Aluminum to keeps costs down. A little galvanic corrosion may in evidence where Aluminum and steel meet.