Cons of using --size-only

[Max Kipness]

I'm doing some tests on rsyncing some Sendmail spool files every 15
minutes to another Sendmail server as a backup. It seems to work well
and efficiently, but I did notice that when users connect via POP the
spool file updates it's time. Using log-format option, I could see that
even though the file was 0 bytes, it was considered changed and
transferring 40 bytes, which I assume should be normal.

By adding the --size-only option, this behavior has changed and only
changes in file size are being transferred. Is there any negative effect
to using this option?

Thanks,
Max