I agree that Logic as soft synth is not a general solution. But for me it works well as I'm using only a limited set of styles (jazz/fusion). So I've loaded Logic with my favorite 16 instruments on different channels. And to change for example the melody/soloist instrument, I simply change the MIDI output channel in BIAB's MIDI Settings to the corresponding instrument channel in Logic. So I don't need to touch Logic at all and can even hide it from the screen like an invisible sound module.

For the future, with 64-bit computers addressing more than 4 GB of RAM it should be possible to build general purpose high-end GM soft synths. NI's Kore 2 seems a suitable tool for this.