Someone correct me if I'm wrong here, but in my experience it takes maybe 1 or 2 clicks to convert an audio file. So when I buy assets that have multiple versions of the same file, it's a big plus and makes me remember them for next time.
The main reason for me, as an indie dev, is that it saves time and helps keep my workflow moving if I'm testing out different music and sound effects or for moments when I remember a great asset that went unused from a previous project but now I'm in a different engine. So that little bit of thoughtfulness in asset preparation is really appreciated.
But that's just me :P