Particle systems are generally implemented with alpha blended polygons so they suffer from the same sorting problems as any other alphas, hence no Z-Write and no depth buffer value. The only few mitigating things that can be done (in terms of sorting) is use additive blending (if that looks good with the specific effect), still it does nothing in terms of depth for alphas.

And you would be wrong. Most games still use a lot of alpha blending effects (smoke, halos, hair, windows, etc.). The problem with using the depth buffer for depth/3D position extrapolation is that alphas don't Z-Write, so the alpha pixels will pick up the same depth as what ever solid geometry is behind it. This is the same problem that plages other post processing effects such as depth of field, depth buffer based motion blur etc...