As I sit here, I have over 4TB of external storage hooked up to my MacBook Pro already, another 2TB standing by, and a few TB more available over my gigabit LAN, but when I'm not at my desk, where exactly do you think all that storage is? Floating weightlessly beside my head ready to jump into action at a moment's notice?
You want me to have to pull out and hook up an external drive in Starbucks or a conference room or wherever?
What do you want me to do when I don't have that option, and instead have to wait around for that critical video, or virtual machine, or whatever, to copy off my external HDD onto my tiny, overpriced internal SSD on which it may not even fit? Even just managing the placement of files and keeping track of which versions are current or not is a pain in the ass, but what do you want me to do when I have 30 seconds to copy 40GB?
Think real world, not some bizarre happyland where those ports on the side of your laptop mean you can do anything with it anywhere.