Just to add, SABNZBD uses par2.exe (just like Alt.Binz) and it already joins these split files automatically. So Alt.Binz wouldn't need more components, just an extra couple of lines of code/switches for the existing par2.exe.
I spoke with the SABNZBD devs about it tonight and they said it's easy to implement, par2.exe just needs directly telling which files to join as it doesn't actually search the download folder for files (unlike QuickPar for example). So you'd need to add to the code/switches:
par2 r -options PARFILE file.001 file.002 file.003 file.004 file.005 etc.
I'm no programmer, but I thought this was worth bringing up
It'd be nice to see joining added to my favourite app