Only under the following circumstances:
- the e-book is on sale
- I haven't gotten around to actually reading the book yet
- OR, I expect I may wish to re-read the book
Generally I don't, but when I decided I finally wanted to start reading the
Dune series, I picked up a copy of Dune on sale rather than reading the leatherback B&N edition that was on my shelf. And then I bought the rest of the series as ebooks.