This is a plugin that has to be installed on the client? Can’t we just not install it?
That's the way I understand it, however, anyone who's got this installed and uses Gmail, can tell whether you've opened their mail or not.
I'm thinking the client in this case is the email sender who can tell that you, as the recipient have opened it. The recipient can't do anything about it and wouldn't even know that it happened. My concern is that it also shows where you are. Potential to let people know when you are away from your house so the thieves can drop by while you are on vacation.
The plugin is designed to be used by the person doing the email tracking, not by his email recipients, who may or may not be using Gmail.
The plugin is part of a larger system designed to turn Gmail into a CRM application. See streak.com.
Demo of Streak email tracking: