One of the reasons why rough scenes are so popular is because 'vanilla' professional porn tends to lack real energy and expression. Porn viewers want genuine reactions from a performer, and to a certain extent, they receive that in rough porn. In non-rough scenes, most pornstars simply aren't convincing enough to make you believe there's any authentic lust or love.
That's the exact opposite of amateur porn. Instead of watching a girl who behaves like a blow-up doll that robotically fakes the occasional orgasm, you frequently get women who genuinely seem horny or love with their partners. It's a totally different experience, simply because they look like they really want to be there.
Romantic porn can work, but anyone making it should take their cues from amateur porn or the mainstream film industry - movies like Unfaithful, 9 1/2 Weeks, Sex & Lucia, for example.