"there is no tangible benefit to DDR3 RAM frequency above 1600 MHz. as this is not a system bottleneck for typical work station or personal desktop PCs."
Actually, AMD APUs have significant graphic-subsystem gains with DDR3-2166 (or any range stepping up from the horrid DDR3-1066 that is usually shipped with cheapo PCs). Intel integrated GPUs don't benefit much, but their GFX performance is horrid (comparatively) anyway.
"so DDR4 and faster frequency hybrid DDR3+ doesn't offer any value for server applications either."
Do note that with increased frequency, your memory throughput increases. Just because current programs don't make significant use of 22Gbps throughput over 14Gbps (most machines only have 4-6GB of RAM total anyway), doesn't mean that NO program could be engineered to do so, especially with knowing there's 256GB of NAND storage hiding in a DDR4 slot (hence the OS support requirement). THAT location is where I, as a programmer, would dump my table cache that couldn't fit into actual volatile RAM, as it's guaranteed to have better throughput and access/storage speed than a spindle drive. Windows could utilize it by copying the whole OS there too. A game could make use of it by stuffing map packs, texture files, etc in there rather than leaving them on a spindle drive. Clustered systems could make significant use as well. We'll have to see. However, no one will design for it if they don't have hardware to test on, nor likelihood of adoption.