Right now, a soft-shell product is generally considered as a piece of light to medium insulation, which breathes extremely well and has good water repellency. It's a piece that can be worn across a wide temperature range, in wind and still conditions, in sun and light rain. In heavier rain, you throw over it a light rain shellnot one of these $425 Gore-Tex suits of armor all the rage a few years agoand you're to all intents and purposes waterproof.
Which is to say, soft shells such as Cloudveil's Veiled Peak Jacket ($165) could indeed start replacing a bulky pile or fleece jacket in many packs. Not quite the same insulation value, but a slight upgrade in underwear (from lightweight to midweight, for instance) would offset that. And yes, it does work. I have a winter cycling jacket made of Malden Mills' Polartec WindblocCannondale's Thermal II ($175). It's comfortable up to 40 degrees. I haven't found the bottom end of the comfort range yet, but know it's good in freezing conditions with a windchill of 20 to 25 degrees. It's not as breathable as regular fleece because it's so windproof, but it's still very sweat-free across a wide temp range.
Price remains an issue, as good pile jackets are $75 less than the soft shells. However, that may be offset by the fact the soft shells have more versatility and generally are tougher than fleece or pile. In general, I think a soft shell piece has a place in most serious outdoor users' gear inventory.