Interesting question. As you say, in the past it was almost standard for fusing (when present) of the incoming mains supply within equipment to be DP; dual fuse holders were everywhere - and, indeed, I still have some (unused) 1960s equipment which has them. A partial answer might be the fact that, at least if you go back far enough, the equipment may have had a 2-core supply cable and a reversible 2-pin plug - in which case DP fusing may have been perceived as a way of ensuring there was a fuse in the L supply (whatever advantage that was perceived to offer! - protection against L-E faults, perhaps). However, that didn't ater the fact that, whatever the polarity of the supply, DP fusing left open the very real possibility that only the N one would blow, leading to a dangerous situation.
That's a good reason to have DP switching (which most of the DP fused 1960s gear of mine did have) or MCBs, but DP fusing still presents a serious potential hazard (just as would having separate SP MCBs in L and N). Even if the N is not reliably tied to earth, the L is even less likely to be
FWIW, I can't understand, either, but it certainly happened.
Even if we did (and I think not, at least in general), I would have thought that having a floating supply would, if anything, be a reason for not having (not needing) DP fusing, wouldn't it?
Ah, the very man
Yes, I know they were banned in the 1930's, but I also know (as you do) that we haven't yet been able to total rid ourselves of them 70-80 years later! 
In the case of centre-tapped (to earth) supplies, there obviously is a need for DP fusing. However, as I wrote yesterday, if the supply is truly floating relative to earth, then I see no real case - a SP fuse in either line would seem to be all one needed. Maybe they used DP fusing with such supplies just in case one (either!) of the lines did somehow acquire an unintentional connection to earth?
I was talking today with a friend about "old people" and how they coped in the bad old days. The concensus was they had more respect for this "new fangled electricity" and used it with more attention to safety. The average person needed far less "supervision" and cotton wool protection than today's Mr and Mrs Average.
There was always the possibility of a phase or neutral being grounded in a house without any adverse effect in that house. Only when the other side was earthed in another house would there be adverse effects.
They varied between extremes. In the case of my grandmother, who didn't die until 1989, that 'respect' manifested itself as total fear, to the extent that, despite many decades of exposure to them, she would never touch a light switch or turn a TV on/off, and continued using her 'flat iron', heated on the gas stove, to the end of her days. In diametric contrast, her husband (my grandfather) would happily connect anything (heaters included) to a bayonet plug and plug it into a lampholder, have amazing Christmas trees of adaptors eminanting from sockets and lampholders, would join cables by twisting together the live conductors and leaving them exposed (in later years, he progressed to 'insulating tape'), and was known to use the pins of a plug to trap bare conductors into a socket to supply an additional load!
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