Well, according to the sources of run-parts (the program that is used in debian cron), the files without executable flag on are silently ignored.
SuSE uses a script named 'run-crons', that gives a warning message: "WARNING: $SCRIPT is not executable, script is ignored !".
Redhat(atleast 7.3; I don't have access to other RH boxes at the moment) has a script that is similar to the debian run-parts, and it also silently ignores the files without -x flag.
Conclusion: It doesn't work without the x flag, and it doesn't necessary inform a thing about it.