Latest source in RawTherapee now supports most Phase One backs, and also Leaf Aptus and Aptus-II backs, probably some of the older ones too. Often the problem is just some missing constants here and there to make a correct decoding concerning colors and clipping levels etc, and this was the case with these backs. Hasseblad support is also in there, I think the support should be quite wide. There's no support for Leaf Credo yet though, as those backs use a new revision of IIQ format which has not yet been reverse-engineered.
If you have a problem with a specific back just let us know and provide some raw files we can probably fix it.
Few medium format users are interested in using something else than the manufacturer's own software, ie Capture One for the most case, as they have all the most fine-tuned color rendering. These backs are generally individually tuned and have lots of proprietary meta-data on board which gives them an edge, and I doubt Phase One is very interested in that other raw developers would have the same possibilities to do the same rendering as they do in their own software. It's tough competition out there, and many want to lock in their users.
However, if you work with say landscape rather than skin tones in studio conditions that fine-tuned color is not at all as important, as color will be off anyway in those uncontrolled conditions, and then third-party software with less tuned color models can be a really good alternative. And of course you can always create your own color custom profiles for those third party softwares if you want to.
Supporting raw formats is very resource-consuming, you usually need to do fine-tunings in the source code for every new little camera model that is released, even if you have a decoder for the raw file format itself. To do really well you'd practically have to buy or rent each new camera that is released to the market and test and do measurements in your lab! Camera (and digital back) manufacturers have really made it a pain for us software developers. If you're a small independent software developer or open source you have to rely on reverse-engineering to rather than licensing expensive SDKs.
The strategy we have in the RawTherapee project is to have rough support for a wide variety of cameras which we get through dcraw, and then add fine-tunings through contributions from helpful users. We can't buy every single new camera released to market, especially not digital backs
I'm really really impressed by the small commercial players like Iridient out there that can handle the pressure of keeping up with raw support and still maintain a profitable product. The resource-consuming maintenance work required is a lock-out mechanism in itself, it makes it easier for the large players like Adobe Lightroom and Phase One Capture One to stay in front.