Why is there such a distinction among writers between historical romance and historical mainstream? I find it strange that the distinction should be made at all. I could stumble towards a definition - I'm sure we all could - but who can apply that definition to a work with utter certainty? and find that everyone agreed with the choice? Very few, I suspect.
Perhaps we should narrow the definitions further. Perhaps science fiction should be strictly segregated from fantasy, and time slip novels should have a whole category to themselves.
Crime - well that could be subdivided very easily. Come to think of it, are thrillers segregated already? Is a thriller found within crime? I don't know because I don't read the genre very often though I do enjoy Ian Rankin's Rebus novels. There I go again, mixing my genres - should novels be distinct from crime?
As an ex-librarian who used to segregate fiction into various shelving categories picked by the whim of the public in my particular library, it all seems a bit of a hoo-hah over nothing. Even the readers never agreed on which title should go in which category...