One of the main reasons while some WordPress installations go south is due to lack of maintenance. (Shameless plug, check out our WordPress Backup & Maintenance Packages.)
If you have some WordPress plugins that have been inactive for a while (more than a month) unless the plugin is seasonal, the best thing to do is delete them.
By doing this you are not only saving web hosting space, but you are also well as optimizing your WordPress installation.
Just like one goes thru their home and clears out what isn’t in use, shouldn’t you do the same with your WordPress plugins?