This is my reasoning as well. If I'd treat a paperback novel the same way as I treat my notebooks, stuffing them in my bag, taking them everywhere, they'd fall apart after a week or two. I select a notebook for durability (among a few other things, like not having squares or lined paper but rather blank, and having a convenient form factor).
However, while I do agree that Moleskine notebooks are absolutely gorgeous, I usually buy other types at about 1/3rd of the price.
Their diary/planner, however, is merely on the high price end for such things, not by a factor of three. And in the years I've used one of those (I use a much tinier planner now), I kept on being pleasantly surprised by their attention to detail in the design of tables, grids and bubbles you'd find in a planner, all of it really well-thought-out. But then, a pre-printed planner is not the same as an empty notebook.
> However, while I do agree that Moleskine notebooks are absolutely gorgeous, I usually buy other types at about 1/3rd of the price.
I used to do that as well, and felt Moleskines were just too expensive. Then I splurged on one a couple of years ago, and never had that feeling again. Don't know if I just feel that I get my money's worth, or that my brain just simplistically adjusted to the price.
Empty pages is also my thing. In general I have four, pocket sized one, A5 sized main notebook, A4 sized sketchbook for drawing, and a reporter style watercolor pocket one (I love the panorama-like landscape format for pictures).