This is actually a pretty good point, and one that I think most people overlook. Hell, if you consider the literary canon, there are a lot of important, formative pieces that aren't necessarily enjoyable to read. Literature professors will acknowledge this, and will tell people that they shouldn't feel compelled to read them unless they're actively studying the subject.
Something is significant not because of its own existence, but because of how it influences its genre. Dune has had a very noticeable impact on a subset of science fiction, and it should be recognized for that. But, to a certain extent, the genre will thrive and continue by moving past it. The works that it has influenced will influence other works yet to come, but some Dune will always remain. You can see this with Tolkien in the fantasy genre--for all that we now have PCs using magic, Metzen orcs, and moral ambiguity, there are still very identifiable elements that can be traced back to Tolkien's works.
This does not mean that anyone who enjoys fantasy novels MUST read The Lord of the Rings. Though, if they don't, they shouldn't pretend that the movies are just as good a source.
It is only necessary to read them if you plan to perform an in-depth study or analysis of the fantasy genre, or--arguably--if you want to embark on writing fantasy yourself.
I say arguably, and stand by it. Demanding that we shackle our creations to those who have come before us will slowly starve our creativity. If one must read Dune to interact with science fiction, what happens when another work of that caliber comes along. Must you then read both works, ad keep both works in mind? If we follow that path, we will eventually box ourselves in with our own canon.