For example, while it's the current top-of-the-line GPU, the GTX 980 has a staggering 72.1 gigapixels/second fillrate, which with back-to-front rendering and/or Z buffer checks, seems almost ridiculously large, possibly even at 4k resolutions. As far as polygon counts go, modern GPUs can do tens to hundreds of millions of textured triangles without a hitch if you batch and/or instantiate them properly.
With forward rendering, the amount of fragments that shaders will run on can quickly become overwhelming, but with deferred rendering, the cost is usually more-or-less constant, depending on the resolution, and we long since passed a point where most shading or post-processing effects can be done in realtime in 1080p.
Either way, the limiting factors nowadays are most commonly draw call counts, and shading costs, both of which are kept relatively low by proper deferred rendering and geometry batching, so with that in mind, is culling more than just backfaces and out-of-frustrum polygons of any substantial benefit? Wouldn't the costs(CPU/GPU time, programmer time) outweight the benefits, a lot of the time?