My daughter just called me a "mouse potato" and I didn't need a dictionary to look that up. (Even though I don't use a mouse with my laptop.) And in most normal circumstances, I can usually get by with an unfamiliar word by context, but...
I voted "Yes" I need one, because:
1) I'm not content to just guess at the meaning of the word.
2) The appeal of an eBook reader is portability and my 2 volume dictionary at home is not very portable and has extremely tiny print.
3) Words themselves are very interesting to me.
4) My daughter is always asking me the meaning or spelling of words, and I want to set a good example for her, by following up on her questions, especially while she is learning and has the interest. (If I could get a decent encyclopedia on my Sony, I'd want, no need, that too!

)
BTW- should I be insulted by her comment?