> I'm just trying to guess what each of that "guru
> options" means and have found two "identical"?> options:
> #R cache repair (--debug-oldftp)
> #R old FTP routines (--debug-oldftp)

The good one is "cache repair" - the old debug code has been removed.

> Are those 'debug' options useful for real-life users
> or they are meant mainly for programmers?
Mainly for programmers - or for advanced users. You can pick up some cache
entries, for example, using some debug functions, or test scan rules. But the
options are not very well documented..