Hi David
Thanks for posting those pictures. The scabbards in particular mirror the style of my piece.
I think you're right – it is a rather vague category, probably created in hindsight. I doubt at the time anyone would have thought of these as "romantic daggers". They reflect the romanticism of that period, I think, and have been labelled that way to identify them broadly to collectors, and perhaps to separate them from later fantasy daggers.
I can only speculate that the reason they were carried by prostitutes is that they were quite pretty and quite dramatic. Mine is a bigger example, but the smaller ones were also obviously easy to conceal, so useful for that kind of work.
I believe the manufacturers of these knives found a ready market among prostitutes, but they were no doubt used more widely too. As Mark noted, probably as letter openers.
This is guesswork. I have no hard evidence of this.
It is interesting that they seem confined largely to France - in fact, to such a degree that collectors elsewhere don't even really know about them.
There are one or two English examples I know about. But the English always ended up imitating the French, didn't they?