As a romance author I use flowers as a prop in my books quite often. Not just because they are considered romantic, but because you can use them as so many different things. Honestly, I've had flowers that communicated telepathically with humans in my stories. In this case I guess they wouldn't be so much a prop as an actual character.
I like using flowers because you can use them in so many different ways. You can use flowers as a symbol of love, as the villain such as when I used the pollen from a flower on an alien planet to kill off some of the settlers. Another time I used flowers to return the memories to a character who had amnesia.