... in some admittedly rare cases, for example where there isn't time to wait for the IAP to timing, the approach with the FRAM or, even the much less appealing BBRAM, is quick enough to get the job done without a multi-millisecond time window for the IAP.

In my particular case, I required an external memory

methinks the real reason is that no IAP (that I know of) will initialize external memory.

I have a real problem visualizing any case where the regular ISP/IAP is too slow, after all, if you know what you are doing, a program update should be a very rare event. Anyhow even for 'small' operations I typically write a bootloader since I would not require the user to purchase a JTAG dongle or whatever, not to mention opening the box to connect the cable. BUT in cases where program updates would be "product supplier performed" I would rely on the standard ISP/IAP

Erik