I would not buy one. I would simply compromise what I keep on the device like I do all mobile devices. Plus I already have a spare 32gb card should I want it. Its a 250 pound bump for what is to me, nothing more than extra storage. I can get the same extra storage via microsd for 50 pounds.Consider the price of a 128GB mSD Card being about $120 when you consider the upgrade from i5/128gb to i5/256gb.
As to the response about the cheaper pluggable dock, I won't be buying a dock initially, I would want to see what my use case would be, whether I need to be able to use kickstand when docked for various angles, but either way, the dock is the one area where I'm happy to pay more than the going rate.