I know what you mean, that it's easy to fool yourself and fall into marketing traps. But in this case of monitor resolution, there is a very noticeable difference (to me at least) between current desktop monitors and retina ones. If in 5 years some company tries to push the trend even further with 8K extreme retina or something like that I'm sure I'll be on your side, but for now, please, bring on the 4K desktop displays!
(Actually, when it comes to TVs I do think we're getting into overkill territory with 50" 4K displays, when for typical TV viewing distances 1080p is fine)
With regards to TV, one of the big effects of HD was that TVs got larger, but viewing distance remained constant. With 4k, at standard TV-viewing distances, it doesn't really make much difference below 100 inches or so. On the other hand, I want one on my desk.
(Actually, when it comes to TVs I do think we're getting into overkill territory with 50" 4K displays, when for typical TV viewing distances 1080p is fine)