Hey,
I could write a tool to manually remove any new fields and values created by gContactSync from an address book if you need or want it. A few extra fields shouldn't waste much space and I have no idea how extra fields affect the performance of Mork databases.
but while gContactSync is uninstalled, these fields are not visible to the Tb Addressbook.
This worries me a bit - is it not a little dangerous?
The fields are visible to the back-end of Thunderbird 3 (only to a limited extent in 2) regardless of whether gContactSync is installed, but since gContactSync adds and displays those new fields there really isn't a way for the user to see those new fields or values without looking at the actual Mork AB file. It wouldn't be too difficult to strip gContactSync down to just the parts that display those fields. All you would really need to do is disable the syncMenu, syncButton and autoSync preferences and you almost wouldn't know it was installed.
When you drag and drop contacts Thunderbird 3 should copy all the fields and their values from the source contact even if gContactSync is not installed. TB 2 only copies the default fields.
What risks were you concerned about?
EDIT - I just read your phone numbers post, so I believe you meant phone numbers in the incorrect places?
Josh