This is the traditional, orthogonal, behavior of the --replackepkgs
and --replacefiles options. Attempting to find interrelationships between
all the options, so that, say, --replacepkgs can always assume --replacefiles
is risky, because  it changes the expected behavior of rpm.  I don't believe
there's sufficient gain to justify changing the existing behavior.

You can, if you wish, create your own rpm options using popt aliases. For
example, you can do
  echo "rpm alias --replaceboth --replacepkgs --replacefiles" >> /etc/popt
and then invoke rpm with
  rpm -U --replaceboth <package>

-U --replacepkgs _should_always_ work without --replacefiles, because what -U
does, is not more than -e and -i one after the ofter (at least it looks like this)

the problem is that rpm isn't consequent,
if the package which is to be updated differs from the installed one (in
version/release) everything is ok
but if they differ only in the files it is not working

from the man page
       --replacefiles
              Install the packages even  if  they  replace  files
              from other, already installed, packages.
                   ^^^^^

       --replacepkgs
              Install  the  packages  even  if  some  of them are
              already installed on this system.
              ^^^^^^^ ^^^^^^^^^

the description is correct, but rpm shows a different behaviour