Most add-ons have needed to be updated for ACT! 2010 and not all have been... but the upgrade leaves all files in the Plugins folder which can cause issues when it starts up.
At very least, the upgrade should check for items in the Plugins folder and give the user the option to stop and uninstall them or continue.
A better option would be to give the SDK a function to register the add-on with ACT! so that an uninstall on ACT! will also uninstall the registered add-ons... as the upgrade does an uninstal, it could then provide a list of those add-ons to the user for them to obtain updated versions and reinstall them