I could see the usefulness of this.
Consider my situation, I frequently am designing electronics and need PDF documents open for the various part-specifications. Reading PDFs is okay on the LCD but it'd be even nicer on eInk (perhaps the size of the DX).
If you had one attached to the side of the LCD display it'd be like having the document hanging there for your reference.
I do not believe the OP wishes to use the eInk display for normal interactive applications as such, rather just for reading documents or other text which isn't rapidly updating.
I'm actually getting a USB->VGA screen adapter precisely so I can do this but for me I'll have to be content with the LCD panel until a manufacturer out there provides a proper generic eInk display (I could get started with one but it's $6000 for the developer kit

).
Paul