I hadn't heard of this before, and am glad that it wasn't filmed. I don't think it would have fit into firefly, especially given that firefly was only 1 season long. Reavers weren't that involved in the series, as compared to the movie, doing this storyline would have completely changed the whole of the series. I don't think it's necessary for any show of fiction to show gang raping, whether it be by humans or other creatures, especially one aimed at families.
That's the thing, though, the show got cancelled before we ever really got to know what it was about. Joss had planned for Firefly to go for seven seasons, he had storylines plotted out for those seven years. You only have to look at something like Supernatural to know how a show can change over the course of many seasons - or even BtVS and AtS.
Firefly might have started off light and fluffy, but so did BtVS. BtVS didn't start to get darker until S2, when Spike and Dru were introduced and Angelus came out to play. So that's Joss' MO - lull us all in with a false sense of security and cut us off at the knees with a storyline like this, lol.
I'm not saying I would have enjoyed a storyline like this - in fact, I can pretty much confirm that I would have hated every second of it and I'm glad it didn't make it to film - but I can understand that the show might have grown and this story could have fitted into that verse, if the show had been given the time.
I would, however, have been interested in seeing the aftermath of that story. We see Inara with he injection in one ep of Firefly, and, supposedly, that vial was filled with a liquid that made her body toxic, killing anyone who had sex with her - which is how she survived the Reavers, and Mal and the crew find her surrounded by dead Reavers.
I would have liked to see how they dealt with Mal and Irana's relationship after that, if the toxin was temporary, or if the crew would have had to try and hunt done an antidote.
I can understand why Serenity was kept to that lighter feeling. Given what we did see of the show, taking a major stop into a darker territory with this sort of storyline for the movie would have been too big a step away from that and the two wouldn't have meshed and a lot of Firefly fans would have been alienated.