Ok, most of the genre separation is useful - it helps people find books of a style they like. But really, there are some marvellous books filed in sections of the bookstore I never visit, that I only find out about because somebody recommended them.

That's part of the reason for the choice of layout in this site - most stuff involved in writing is common to all genres, but there are still sections for the stuff that is specific to the genres. Time travel is obviously a science fiction-only thing (unless you're Michael Crighton, in which case it is mainstream - see his book Timeline, for example).

Anyhow, since this article is in 'Musings', there isn't really a question, but you're more than welcome to post your own rambles on what you think of genres and their segregation.