Apple will offer only so much free cloud storage (5GB, if I remember right), and I think they're only going to store content like music, books and apps, so you'd still need some place to back up everything else, even if you trust them to store music, books and apps.
I'm not the trusting sort, so I back up everything, lol.
If you go over Apple's cloud storage limit, you'll have to pay, or get free storage elsewhere.
Also, cloud storage relies on an Internet connection (whether wi-fi, my-fi, 3G, 4G, LTE, whatever). If you plan to stream or otherwise draw a lot from the cloud, you will need a data plan with a big allowance. And if your data speeds are slow, that can affect your quality of music, video, etc.