I do hope that the crossbows load really slowly, like real crossbows. If they're just gonna be sideways bows then adding them becomes purely cosmetic.
Bethesda states that apparently the pros and cons to X-bows will be along the lines of:
Pros- Can be stored on your back while loaded, for a quick shot when you need it and every shot with the crossbow will have a chance to stagger the enemy.
Cons- The crossbow will take longer to load and (possibly?) be less accurate/or less range
So far I like the look of the steel crossbow they've shown us, it's small and light looking which matches my rogue perfectly. But I also hope to see some massive portable ballista type machines as well.