This is an small piece of software (source included - made with autohotkey) that can be used to copy updated addonsfrom one WoW installation to another one, without adding any addons that are not already installed.
I use it send new versions of addons I updated to other machines on the local network. The benefit of using this program above doing it yourself is that you don't have to check which of the addons you've updated are also installed on the other machine. For the techies: it checks the last modified date of the addon folder.
Visit the download page to get the latest version.