[citation][nom]randomizer[/nom]It's worth installing if you want to reformat, but it's a waste of time and disk space installing a cumulative security patch over the top of the security patches you've already got. That's basically what most Windows SPs are - cumulative patches.[/citation]
First, Microsoft recommends you install service packs through Windows Update, so it only has to download and install what you do not already have.
Second, even if you download the stand-alone version, you're not going to be doubling up on updates.
Third, service packs, while including all updates since the last service pack, typically contain OS enhancements and improvements that you are unable to otherwise obtain. Running with all the latest security updates isn't the same as running the most current service pack.