When I began snorkelling back in the early 1960s in England, full-foot fins were the fins of choice for many UK scuba divers. They were also much more expensive than open-heel fins of the period. Many divers owned two pairs of full-foot fins, a smaller pair for the indoor pool and a larger pair for open water to accommodate booties. When I purchased my first pair of Typhoon Rondine full-foot fins at my university diving club in the mid-1960s, I found them blissfully comfortable after years of using uncomfortable open-heel fins. I vowed back then that I would never revert to open-heels and I never have since. I still snorkel two or three times a week in the cold waters of the North Sea off the North East coast of England, wearing full-foot fins over my suit. The bonus nowadays is that full-foot fins are cheaper than open-heels too.
Open-heel aficionados often assume that full-foot fins are always worn barefoot or over the thinnest socks. This may be true of divers who only dive in tropical waters, but it certainly isn't true of every full-foot fin user. What about freedivers, for example, who almost always prefer full-foots to open-heels?
Some full-foot fins are even designed to be worn with boots: the full-foot Apollo Uni Bio split fins come to mind. What matters is the capacity of the full-foot fin's foot pocket (click the images below to make them bigger):
which is why any prospective footwear should be worn when trying on any type of fins for size, whether open-heel or full-foot.
Over the years, the choice of open-heel or full-foot has often been dictated by the prevailing fashion. Open heel just happens to be in vogue at the moment. At other times in history, diving fin manufacturers have made full-foot fins with huge foot pockets to accommodate even dry suit boots. I'm thinking here of Cressi's Uni fins with European foot sizes in the 50s (US sizes 15 and well over) for the Unisuit dry suit market in the 1960s/70s. There's also evidence of national preferences at work, e.g. European divers preferring full-foots, their American peers sticking with open-heels. After all, full-foots were invented in Europe.
Yes, personal preference can still play a large part when contemplating what type of fin is appropriate for a given individual and in a given context. We live in a diverse world, with different priorities and different preferences. That's a good thing.