The big advantage that I remember for moving over to KS3.1 was that it was required for the spectrum analyiser on one of the WinAmp clones to work.
WooYAY! :afro:
I've just dropped back to KS3.1 and WB3.0, after using 3.9 for the nearly 5 years (I think it was 2001 when i got it). My old 3.0+goodies set-up was much nicer, especially when used on TV (like I have to now due to lack of space).