I do this, but for paper books I want which are likely to become unavailable once the initial and sometimes only print run is sold out (reference/artbooks in specialized categories can have a surprisingly short shelf life, so I stock up if it's a subject area that interests me and the book looks good enough to keep upon flipthrough/Look Inside).
And I stock up on French translations of my favourite other-language sf/fantasy titles because those things tend to disappear fast, rarely to be seen, much less reprinted, again.
But not really for e-books, which are mainly fiction and usually available to me via other sources anyway (library, used). I do stock up on them during sales, when say Fictionwise has a deep-discount coupon and I can pick up the new backlist releases of $AUTHOR whom I like since the last time there was that good a promo coupon.
But's just discount splurging and I don't think I buy them for the sake of buying them "just in case".
Though it turns out I really ought to have done in the case of Mike Resnick's novels on Fictionwise (now nearly 40% more than the original MultiFormat FW pre-coupon price and DRM-ed from the new publisher), and the Subterranean Press stuff on Baen (withdrawn from catalogue a mere week before I was planning to splurge
).