Do people tend to actually pay you to develop proposals? Or is that just your way of turning down RFPs?
I make this same observation every time the spec-work argument comes up (if you haven't picked up on it, I'm also not against spec-work):
When we were just getting started, lawyers from WSGR and another firm spent literally hours on the phone with us, for zero money and no promise of ever being compensated, presumably because the fact that they're willing to be that cool to work with costs them very little and makes them incredibly attractive in the long run.
I guess the main reason this works for us is that we prefer smaller projects. We don't aspire to being a huge consulting company / agency taking on bigger and bigger projects. We like working on small projects that grow into bigger things.
Just be aware that you're not only restricting the size of the projects you take on, but also the selection of clients you have to work with. Some very excellent projects will require competitive proposals for you to get in the door.