Windows File Protection and System File Checker

Firstly, is the scope of SFC wider than that of WFP? Otherwise, what is the
point of SFC, since "bad" files should be picked up by WFP running in the
background? I would like to understand this.

Secondly, I believe that it matters exactly which version of CD (Home/Pro,
upgrade/OEM/retail - are there others?) is used for a repair installation.
Does this also matter for use with SFC? I know you can copy the I386 folder
to the hard drive and point to it there so as to avoid needing the CD. But
do the I386 folders vary between different version CDs, or is it just the
installation programs that are different?

Thirdly, it is said that if SFC replaces a "bad" file with an original that
is now out of date, this will be corrected the next time you visit the
Microsoft update site. Why doesn't the update site just replace "bad" files,
thus automating the correction process and making SFC redundant (at least
for computers using automatic updates)?

Thanks,
Stephen