The ready access to dictionaries is one of the top three reasons for using an e-reader (portability and easy acquisition of books are the others). As a writer, I'm interested in the structure, vocabulary, and grammar of what I read as well as the story. I do have an extensive vocabulary but using a dictionary to look up either words I don't know or words that are being used in an unusual way gives me a richer understanding of English. As a pedant, that's important.
The last book that I read extensively on the e-reader, before my wife permanently commandeered it, was Joseph Conrad's
Nostromo. Mr. Conrad used English words I didn't know, used the ones I did know in uncommon, secondary or tertiary definition ways. He also had a fair bit of Spanish, some French, a touch of Italian, and, maybe some German in there. I was using a Sony T1 and the multiple multilingual dictionaries really were great. Having access to all the dictionaries made it a much richer experience.
On the other hand, I often use my phone as a reader on my commute to and from work and, since the commandeering of the T1, in bed. None of the reader apps I use integrate as well with dictionaries and they need a network connection to access the dictionaries so I'm out of luck when I'm on the subway (aka metro). I do have some dictionary apps on my phone and I'll use those if I really want to learn a word, but it's more cumbersome, going out of the reader, opening the dictionary, checking the word, then going back to the book. I was so impressed with the ease of using the dictionary that I specified to my wife that I wanted a T1 as my Christmas present as the T2 has fewer dictionaries.
Frankly, I would pity anyone who didn't see the need for dictionaries in a reader. They must be reading stuff that provides no challenge, no stimulation, and no new ideas.