This little add-on (yes it is an add-on packaged with Thunderbird) is, contrary to some suggestions not a remote control for Thunderbird. It is at the most basic a feedback mechanism between the user base and Mozilla. There has been a widening gulf between what the user community want in their mail client and what the developers are delivering. Much of this gap is because the developers just don't know how users use the software. In steps test Pilot for Firefox. That is right, Firefox has had test pilot since version 4 and it has proved so valuable there that the Thunderbird people have copied the idea (and most of the code as well I would guess).
For the full story, the MozillaLabs Welcome to TestPilot is short sweet and to the point. Sure it is all about Firefox, but as it is the same thing in Thunderbird go with the flow.
At the time of writing this the test pilot program is just really getting started, even though the Test Pilot has been in Thunderbird since Version 9 Whilst you can opt out of Test Pilot, simply be disabling or removing the add-on, I would encourage everyone to keep it. The only way the developers are going to get lots of feedback about the way Thunderbird is used, and some of the telemetry on what is going on when things run slowly is if we all do our bit and send data about the program back to Mozilla. There is no privacy concern here. just an honest attempt to get information that can only improve Thunderbird.