With pattern-based sequencing, plenty of presets and a powerful analogue-style synth, Cakewalk's affordable soft studio package could be a valuable compositional tool.
Disco DSP's additive synth plug-in offers both high-quality resynthesis, with up to 256 partials per note, and a novel 'spectral display' for editing your sounds.
If you could have one VST Instrument that emulated a hardware workstation, giving you enough polyphony to produce a complete arrangement with a fairly standard computer, would you be interested? Steinberg and Wizoo think you would...