I appreciate that such a format would make parsing easier for you.
We wouldn't be able to make that change generally, because we need to support backwards compatibility. You could submit an enhancement request at Zimbra Bugzilla (http://bugzilla.zimbra.com) to add a new variant of GetContactsRequest or even a new SOAP request to do what you desire. I'll be honest and say that I personally wouldn't be a fan though. It would make our WSDL significantly larger (although perhaps slightly easier to consume) and add an extra step to enhance the WSDL each time we add a new attribute (unless we invest a fair amount of effort in creating a new mechanism for keeping the attributes and WSDL in step automatically) The fact that we haven't chosen to create such a format for our internal use suggests that we are happy enough with the current format from a parsing perspective.
Another approach you could consider is to use the REST interface to get the contact information either in csv format or vcard format. There are libraries for many programming languages which help with processing both those formats.
Another possibility, if you're using JAXB objects, you could add methods that process the "a" list to produce the information in a form you are happier consuming.
All the best,
Lead Engineer - Server
Zimbra | Community & Collaboration