Great way to use this would be to have a COMPLETE ammo factory supplying your Spidertron with rockets for combat.
You could also have a production line crafting repair packs and Construction/Logistic bots for use in your roboport.
In short, you get a mobile base (or Moving Castle) to go out and scout for more raw resources for your main, stationary base.
Edit: I really like the idea of the factory showing on the spider's back ... but you would need to create a completely new graphic for that.
It can be done ... Check out Curcuitissimo {the combinators and wires actually show up crazy small in the entity you place.