Activate site wide vs. activate by user


I’m upgrading my WPMU Dev plugins and it has raised a question.

Plugins like Forum, Popover, Subscribe by email etc are plugins that users can choose to use or not. If activated, they add some menu options etc. But should these plugins still be activated sitewide to work, or can I leave them to be activated or not by the individual user?

So if someone wants to have a forum, they can just activate the plugin and set it up. If they don’t want to have a forum they can leave the plugin unactivated and not see the menu options connected to the plugin.

The later option would make it easier to make i.e. the popover plugin into a supporter plugin.