> This is exactly what MIDI is. By installing a MIDI-pickup, you can just
> use your guitar as a MIDI-controller.

Exactly.
For drums you just map out your drummap so that certain notes on the guitar trigger different parts of the drum kit. For bass, you transpose everything down by an octave. For most keyboard parts you use the hold pedal to sustain them. The only time you have to resort to a keyboard is when a guitar isn't practical or when a part triggers too many glitches.
You can use the guitar synth to fade in synth or string parts behind your guitar playing when you play live too.