Does it help against malice the way it does against rot?
And anything else? I've been told wither is also a "wasting disease", but I feel skeptical.

The only thing I ever really noticed it helping with was rot, which isn't that big of a now. I would guess it helps with power word decay also, and I think Ice devils might still have old school rot.

One time after the changes, I accidentally targeted myself with rot on a shaman, and it hit me with old school rot as well.

When I had it in the past, rot still used to hit me and I still has to take anti-rot measures. As a Drow it just used to mean that by the time rot wore off I'd only got a -4 con modifier instead of a -20 or so one. I'd always keep max con from a rot-suit but the save each tick was better with the edge.

Assuming that translates to malice, you'd probably still get hit with the malice, but so better at avoiding con loss on the tick? Probably still need to ensure your con is high though to give yourself a shot at making those saves.