A very nice feature of my GMail account is that you can set the reply-to email to be something else than GMail.
That solves the problem of data ownership since it is easy to maintain a conversation both on GMail and on my existing email system via the following system:
Home email forwards all mail to GMail and Office
Via reply-to I can respond via GMail and still have the email returned to my old email address
The forward to the Office makes sure that the Windows "You've got mail" envelope works for my personal well as well as office email.
Obviously I would prefer email to work differently: Ideally email reply-to could indicate an entire route for the response to take so that all the systems that archived the inbound mail also archived the response transparently and maintained consistent as a result - without me having to use more than one client for the different services.