VG, I hope you don't mind if I extend your excellent description just a little bit:
Yes once you unhide, unlock it, and clean it out, you can use the extended rom as a small (9.9MB) storage card - with the advantage of faster speed and quicker wake-up time than your mini SD card is likely to have. This makes it useful for today plug-ins etc - I use it to install almost every app that's not in the rom.
Your point on including the customizations in the base rom is an excellent one - unfortunately, I don't think any of the kitchens [or roms] really document which customizations are actually incorporated, given that some of them (especially in the Sprint rom) are really undesirable. I've copied the full contents of the Verizon extended roms over to my SD card just in case I want to run individual cabs - of course I never do.
Finally - the big drawback to using the extended rom as extra storage like this - it can cause problems if you routinely save email attachments to the storage card. If the extrom is visible, then pocket outlook can't see your storage card at all and wants to sync the attachments to the extended rom instead. Many people have encountered out-of-memory errors in this situation since 9.9 MB is just not big enough for their attachments. The workaround is to use the "mounthidden" registry tweak to rehide the extrom. Although re-hiding it can make it a pain to get to, programs you've installed there will still run.
http://forum.ppcgeeks.com/showthread.php?t=8535