The idea behind a refillable cover isn't ours alone — it's basically the whole premise of the traveler's notebook world, and people who've lived with one for years tend to make the same case for it.
3 Reasons I Love Using the TRAVELER's Notebook for Travel Journalling lays out why a single leather case with swappable inserts beats buying a new notebook for every trip, project or mood — you keep the cover, the story inside just keeps changing.
That's the whole idea behind ours too.