Maybe because of all the extra-stuff, FMCB provides?
You missed the point, that FMCB itself is not responsible, for playing Backups.
But I agree in your question, if it was related to ESR.
...but even than... Some Chips might not work with some games and they probably still can play it through ESR.
(Assume a Slim V14, with a crappy chip. No HD and the games, or applications like USB-Advance, which won't play with this chip,... With turned off chip and FMCB [nearly] all remains possible,...)
FMCB is a very good enhancement for old chips (if compatible), which e.g. do not have any DEV.-Mode, for those who want to have Fastboot for all Media (is there a chip, which support it for all?), hacked OSD.
I still think, we will see Browser 2.0, or BB.Navi running, due to this project.
Browser 2.0 is even more important yet, than BB.Navi (at least for me).
FMCB could provide extra-features to every PS2, with which it is compatible.
PS2 V7/DMS3 V2 (FW:2.4b7); Seagate Baracuda 200GB
PS2 V7/CC1.0 (FW:34 hacked v2 BM:2.1.6); Maxtor DiamondMAX9 PLUS 160GB
PS2 SCPH-30004R; NoMod+NoLaser
3xSony MC 8MB
MAX/Datel 16MB with Boot-CD
Custom FMCB 1.8b+ Beta-Build, my AIO 0.5, Sony&xRhino-Linux