For me, the benefit to a cross bike (if you are not racing cross) is the multi-functional nature and being able to add fenders, racks, etc. I'm not sure carbon cross bikes have that ability. Maybe some do. However, a mixed purpose cross/climbing bike sounds like a not ideal mix. My road bike IS my climbing/descending/flats/fast bike. If I wanted to ride gravel roads or anytime I need to carry more stuff I'd take my aluminum cross bike.