I was going to suggest a cross bike, for being more tough for all kinds of roads, dirt etc, but wouldnt any kind of cross bike not be that different than your other bikes (in terms of performance, weight etc)?
I am assuming you would like a Roubaix type bike for the fun factor of a fast, light road bike? A number of cross bikes would certainly be more flexible for going on rough stuff, wider wheel choices, but still be fairly light and sti equipped--but may not be t h a t different from your present bikes, especially when you factor in riding whatever you choose "unloaded".

That said, I would think that a cross bike would be a lot less to worry about being rough on. Plus you could put fenders on with 28s as well, perhaps not possible with the Roubaix (maybe not an issue for you).