Not necessarily. I use a 256GB 13" rMBP and had a 256GB MBA before, but I got along just fine with a 128GB model prior to 2011. I would still if I didn't need to run a few Windows applications. With USB 3.0, fast external storage is cheap and easy. There are 64GB and 128GB flash drives now that outperform typical hard drives.
3/29/13 at 8:19pm