Oh, I've got several of those. I've got a NAS too with a TB of storage. The reason I upgraded my MBP to the 750 GB drive (it originally had 120 GB, similar to the current MBA) is because I was tired of plugging/mounting external drives for some collections of files, or shuffling files off the drive to make room for others. 750 GB (I wanted a TB drive, but they didn't make one in 9.5mm form factor at the time) is enough that I can do what I need to do without worrying about whether I'll have enough space.
I'm thinking of getting a 13" MBA, and if I do, it will be a lovely travel/mobile machine. But it will not be the main storage location for my files -- which will continue to be kept on the network, in the cloud, or on my main workhorse computer, which will continue to be the MBP.