I agree - I've looked over the 'Firefly Media Server' forum, and read the topic 'What is the firefly media server?' and I still don't really have any idea what it is or why I should use it over Windows Media Connect - which I prefer since I feel it is the server least likely to cause other problems. After all, it's practically bundled with Windows XP so you wouldn't write an application that didn't co-exist happily with it.
If you only have WMC compatible media and you're happy using a PC as a media server then WMC will be just fine for you. As long as you don't run into any of the bugs of course
... However, there are servers that perform better, support more diverse media, run on numerous hardware platforms and IMHO are more stable. Firefly, in its guise of mt-daapd, is one of those. It all comes down to your individual requirements.
Having said that, I recognise that Firefly will just be working with normal uPnP stuff as do the other servers, so I guess support for the others can't be removed - but I also know that Soundbridge works better with some servers than others, and I would be disappointed to see it become optimised for something other than Windows Media Connect.
Firefly uses the new Roku proprietary RSP protocol in addition to Apple's DAAP protocol (for Roku branded equipment). It doesn't use UPNP. RSP gives Roku the opportunity to deliver functionality (I'm hoping for synchronised playback) that can't easily be achieved with the existing protocols such as DAAP or UPNP. Don't forget that Roku don't control the specification of DAAP or UPNP.
As for the client side, the Soundbridge currently supports all three protocols and Roku appear committed to all of them so I don't think you need be concerned about UPNP/WMC compatibility. However, I think we will see a scenario developing where features become available on the Soundbridge that only work with RSP servers simply because RSP is the only route Roku have to implementing them.