I find the assignable outs are good for if I want to print some hardware FX on a sound. I have the Assignable outs routed to the patch bay where I can then run it through some of my external rack FX units, ensoniq dp/2, pedals, or whatever.
This allows me to assign only what I want to add FX on to the extra outputs. Then route them through the FX which I can hear while the whole mix is playing (but with fx only on the sounds I want). Then I record that back to the inputs as an audio sample so that I now have the effected version and the dry version as samples to use in my project.
I really think they are best for that, when you need to process something externally as an individual sound rather than the whole mix.
It would be cool if you could use the assignable outs as a send
