Just for clarity - Sonar always froze all tracks of the SAME synth (ie, multi-out synths).
This new function (based on the benstat add-on) sounds good though.
But as for the need for freezing ... I almost never need to freeze anything but I do for other reasons ...
Sometimes it's much more convenient to work within the audio-only domain. And, there are different approaches to mixing/editing when staying in the audio-only domain.
Another advantage of doing so is consistency. Some synth patches change over time (like with LFOs, etc). Freezing can keep it consistent.
And, of course, one can easily unfreeze if need be (with the caveat that any edits/changes to the audio portion will be wiped out).
It's nice having all the options, of course :)