I get that. I also read many old books. Usually I can get context out of the sentence itself, but i'll occasionally bust out my phone. I see why it's nice, I just don't see why it's a top-tier feature.
Each their own of course. I can imagine if I was looking up 2-10x as many words as I am now then the convenience would indeed be much more important.
I suspect if you compare the usage of dictionaries among paper book readers vs ereader users, the latter use them more often - probably at least partially because they're so much easier to use. I suspect the incidence of not understanding a word would be pretty much the same.
For myself, I read a lot of older books with archaic and niche terms, so it's practically required to be able to look things up if I want to really understand what's going on.
(As an aside, I looked up a definition on Google just writing this to ensure I was using a word properly. :P)