No multitimbrality. Working with a sequencer or other VST/AU host, you load one instance of VI per instrument you're using. But you have all the articulations you need for that instrument on one MIDI track, which makes all the difference concerning the tedious act of getting your mock-up together ... No deliberating whether you should put that sequence into the legato track, no juggling of notes between tracks.
The stand-alone is good for live play, where you rarely play more than one instrument at the same time. And you could load matrices of different instruments and keyswitch between them to get several instruments during one performance.