I can understand wanting or needing to turn a midi track into audio... If it needs to be transported, the possibility of the plugins being available at the new location is shaky... (ya think?) ;-)
So I usually turn all tracks to audio at completion whether necessary or not... The original midi and plugins still exist anyway and I can always go back to them...
I actually do this all the time when working with my external synths... I record the midi on a midi track while recording the audio output of the external synth as audio... So I don't have to turn that/those synths on evertime I work on the song... I can easily go back and render a new version...
But there are always "situations" so I'm sure there is somewhere I chance to need/want this as an one-pass function for softsynths...
Keni