GMail has no notion of a retired email address. For instance, say a friend is changing jobs, so her work email is no longer valid. If I remove that address from her contact record, then anything she has ever sent me from work won't show up if I search for recent conversations with her. If I leave it there, it's polluting my contacts list with defunct addresses that I will never use again. The best solution I can think of is to rename it from "Work" to something like "DoNotUse". It's ugly, but it maintains the address association for historical records while reminding me that this is no longer a valid address (although it will only remind me when I look at my contacts, not when I type a name for an address).
What would be better, if GMail is going to hold all my email for all time, is to be able to mark email addresses (and perhaps other contact details?) as expired, while maintaining their association with contacts.
Mokalus of Borg
PS - This might not come up if I didn't have six years of email in archive.
PPS - I'm pretty sure I've dissociated other addresses from their owners, losing email along the way.