Having a dictionary feature doesn't mean that it is going to be useful. If it is not easily accessible, then, it almost like it is not there. For example, for Kindle DX, you will have to navigate to the word in order to use the dictionary. If you tried it before using Kindle DX and using PRS-600, you know what I mean. In the same context, even you have a book dictionary in your house, doesn't mean that it is helpful to you. Most of the time, you don't use it because it is too much hassle.
As an engineer, I know that feature accessibility (or user-friendliness) is an important criteria. If you design a product that has many features but hard to use, your product will not be popular.
For the record, I do know that Nook has a dictionary.
Thanks eGeezer for pointing out that again.