The "real stuff" involves conversion from UART/USB to SPI, which involves certain intelligence (mcu). And I refuse to call such device "a cable". Still connected to the target by 3-4 wires (+gnd+vcc), hence ISP. If you refuse to call such "a programmer", please, suggest some better term.
ah, you are referring to "remort intelligence" as opposed to "bit banging". Had you just said so earlier.
Is the "bit banging" of what you call SPI programming one of the things that got troublesome after Mr. Gates decided that timing was his to control (Post -98)?

PS. RE standalone programmer: I doubt you will be able to build a "one-button-box" for field upgrades based on FlashMagic for under $100 so what was your point?
The point is that whatever the cost of a "one button box" ($0 if you use UART ISP) there is no savings with homemade software.

Erik