> Modern GPUs actually allow 16 bits per channel of precision when rendering, but it comes at a significant cost - various features no longer function, memory bandwidth is devoured, etc.

I'm not sure that this is true. It's extremely common for games to render lighting texture maps with HDR render targets. 16 bit-per-channel render targets are often used to accumulate excess light. The excess light gets processed by successful full screen pixel shader passes. These passes generally include blurring and flattening into a normal 8-bit-per-channel texture on the swap chain.

Furthermore, multiple render targets have been common since before the release of the Xbox 360. Deferred shading engines come into and out of popularity on what seems like a biyearly basis depending on what visual style is popular in games currently.

It seems to me that modern hardware is quite capable of greater color range, but the most demanding applications, games, would rather use that extra video RAM and fill rate for storing and processing surface normals, lighting scalars, material properties, etc. Desktop applications always lag behind games dramatically. Hell, browsers are just now starting to see some of the benefits of modern GPUs.