There are some reasons:
1) People like to buy the best for the lowest. At least that's my case. Why? Maybe I'm not using at 100% right now, but if it cost the same, why not? It's an intelligent decision IMO. Also it will become obsolete latter.
2) I agree partially on your argument. Today an average computer con handle almost everything without too much difference (GENERAL statement, continue reading please). I remember the days of 386, 486 and Pentiums. Those days computers really needed improvement to keep pace with user demands.
3) BUT, your argument is not right, because it depends on what you are doing with your computer. Example:
- Playing swapmines, solitaire, internet, some Office, maybe some good old games (not really demanding): of course, a crapy Celeron 1.7 with SDRAM can handle, as well as a P4 2.8 with RDRAM 1066 and a K6-2 400 with EDO RAM.
- Playing latest games, 3D stunning effects, with lot of AI, all real time, etc: you will notice a BIG diference. Obviously the video card is crucial, but don't forget that most of these games are also CPU hungry.
- Rendering 3D, ripping videos, data analysis, etc: again, you will notice the diference. 16% faster is doing something that take 6 hours in just 5, and if you are doing quite frecuently, it's a lot. Bandwidth aplications really run faster with higher CPU and better memory. So choosing the better one is worth the effort.
That's my point of view.
DIY: read, buy, test, learn, reward yourself!