Great philosophy is not always easy. Some philosophers – Kant, Hegel, Heidegger – write in a way that seems almost perversely obscure. Others – Kierkegaard, Nietzsche, Wittgenstein – adopt an aphoristic style. Modern analytic philosophers can present their arguments in a compressed form that places heavy demands on the reader. Hence, there is ample scope for philosophers to interpret the work of their predecessors. These interpretations can become classics in their own right. While not all philosophers write obscurely (eg, Hume, Schopenhauer, Russell), many do. One might get the impression that obscurity is a virtue in philosophy, a mark of a certain kind of greatness – but I’m skeptical.
The problem with the essay is the problem that usually plagues discussions of clarity in philosophy -- the lack of any real account of what the distinction between clarity and obscurity actually is. One sees this in the claim that Hume writes clearly, which is not something that is clearly true -- he has often been accused of obscurity, and he certainly has a trick, which any Hume scholar comes across occasionally, of writing a sentence in such a way that it can be interpreted in at least two very different ways. And Hume himself came to think the Treatise was not as clear as it should have been, despite the fact that it is what we all study most closely. (And it's worth always reminding people that the way Hume writes was considerably different from the way he spoke -- he was laboring under a handicap, like someone who speaks only Portuguese trying to write in Spanish. Scots and English were much farther apart in the eighteenth century than they are today.)
Clarity and obscurity cannot be distinguished solely on the basis of the features of the text itself -- for instance, a highly technical treatise may in fact be quite clear to someone who understands the terminology. It's not something that's purely a matter of authorial intent, since authors can fail to communicate things they are honestly trying to make clear. It's not something that can be determined with regard to readers in general, since that would make anything technical unclear. If we try to determine it with regard to specialized readers, it's unclear how Hume gets counted as a clear writer, given the sheer variety of interpretations of Hume among Hume scholars. Frankish occasionally uses 'hard to understand' as a gloss on 'obscure', and Hume, while very rewarding to study, is at least arguably hard to understand.
The distinction between clarity and obscurity can certainly do some genuine work in assessment; but if it's not simply a personal assessment (what's clear or obscure for me at this time given my background) it is best if we not assume that everyone knows already how the distinction is to be made.