The problem is it's not convenient to carry external drive. How difficult is it to update internal hd?
Well the LaCie Thunderbolt Drive is light, rugged, and bus-powered so easy to toss in a bag if you need it. 1TB of storage on the go.
But, the things I use day to day all fit fine on the built-in 128GB SSD. And since I can't afford nor want to pay the astronomical cost for a 1TB SSD, I'm able to deal with the slight inconvenience of storage management. Think about how often you'll really use the stuff on your machine buried away and how much it's really worth to have it all on one internal drive. If the convenience of accessing your stuff at all times (even if most stuff may get accessed once or twice a year) is worth the money for more storage, than go for it. It wasn't to me so external storage made sense.