Team tab is removed every time Excel 2010 is opened

For Excel 2013 x64 and Visual Studio 2012, I had to change HKEY_CURRENT_USER\Software\Microsoft\Office\Excel\Addins\TFCOfficeShim.Connect.4\LoadBehavior to 3 (Load at startup).

I had previously disabled the add-in via the Excel COM Add-Ins dialog box. This changed the add-in load behavior to "do not load automatically". Attempting to re-enable the add-in through the dialog box now only changes it to "load on demand", which is only good for one session of Excel. There is an issue logged in Connect for this. Changing the registry manually works around the bug.

By chance, I re-enabled it (after having to manually re-add the tab again).

Excel > File > Options > Add-Ins > Manage > Disabled Items > select items > click Enable.

As easy as that.