I just had this conversation with my hubby about this very thing. I'm done writing one story where I just had a basic vision, but I had so many stumbling blocks with plot and the way that things happened, I decided to do things differently this time.
For the follow up novel (not really a sequel but a stand alone with characters from the first) I had a basis of what I wanted to do. I have to stick with how I ended the first novel, so that much was set at the beginning. This time I'm going to carefully outline the plot, so that I don't have these durr moments and have to erase entire chapters because of implausibility.
I think that outlining is important in my case because my stories are mystery/suspense and there's a certain amount of audience participation in that. But it got me thinking about needing an outline for a basic romance with no subplots except maybe a jealous lover or a sick friend, coming out issue etc.
I would think that basic romance formulas could be written in fly-by-the-seat-of-the-pants mode, while anything that has plot elements that rely on research/plausibility etc, should have some basic (if not detailed) outlines.