i have not had problems with customers_default_address_id getting scrambled as part of this module. my customers who use this module also use FEC.
i have one customer who uses OPC along with this module, and that customer would occasionally have problems associated with their address book entries being screwed up. i think the latest version of OPC addressed those problems.
on sites where you are seeing that problem, i would look to see if admins had placed an order for a different customer just prior to the customer getting screwed up. a difficult bug to track down, but that has been my only experience where address_book_id entries getting out of sync.
if you think it is related to this module, there is only 1 spot where this module updates the default address book entry and i would think that could be commented out with no ill effect.
best.
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