I agree that on a modern arranger, trying to create a decent style, or even a single decent style PART, from scratch - is all but impossible unless you are a studio-grade performer and a top-notch computer head to boot. That said, one feature I do find particularly useful is the "Style Assembly" that Yamaha gives on the PSR-3000 (etc.). Taking existing parts from various styles and combining them in different ways provides immense flexibility in customizing styles to fit your particular needs.

Ironically, the custom style creation features were more relevant a decade ago, when the styles and parameters were simpler - the gap between a stock style and a user style was not as noticeable.