Why so Deragatory?

That said, I'm writing my senior thesis on detective/mystery fiction, a category of what is often referred to as "genre fiction." After doing the readings for tomorrow's class, I'm starting to understand that "genre" is often used as a deragatory term, something that classifies unneccessarily and unreasonably when more maleable or indistinct classifications would be more appropriate. If genre refers to something which falls into a particular category, something which follows a certain series of guidelines and contains a certain number of recognizable features, then how can anything which has a defined "genre" be in any way original or innovative?

I like to think that formulaic constructs, such as the ones which lead to something being classified as belonging to a particular "genre," allow more room for focus on individual details. If, for example, a romance novel (I do mean romance with a lowercase r, rather than the Romance of the Romantics) follows a certain pattern when it comes to plots, and involves certain different kinds of characters. Each romance novel, or so I have been told, neccessarily contains one of several available stock protagonists, and several available stock romantic issues. On it's face, it seems impossible that a romance novel could create or invoke anything original. But the formulaic structure of the plot allows for more focus on detail, so that although one character may fit into a certain archetype, the author has the opportunity to flesh out that character and create a very original, unexpected sort of person. Formulas do not prevent originality, they simply confine it to certain areas of the text. I wonder if I'm coming through clearly, here, I confess that I'm not sure.

A note to end on: The second article we read for tomorrow's class deals heavily with the idea of the genre as a historical construct, ie "the novel" or "the poem." Genre fiction tends to fall mostly into the category of the novel. I've rarely (read: never) read a mystery/detective novel in verse, for example. How come?