My wife and I merged our contacts list yesterday. I was wary of the consequences, but it was the easiest way to get our shared info synced and neither of us minded the bloat from getting each others’ names. We sync like this: her phone <-> her Yahoo acct <-> Address Book on our shared Mac <-> my Google acct <-> my phone. Address Book is the bridge in that data flow, because it allows synching multiple accounts (although only one Yahoo and one Google, not multiple of each).
I was surprised to find that Gmail’s Contacts sub-app doesn’t have a way to share a subset of your contacts except via import/export. We’ve been very happy with Google Calendar’s ability to share calendars. I think a mesh of shared and guaranteed-up-to-date contact details is something that social networks are going to offer in the near future. Google is partway there through G+. LinkedIn has been a really good source of up-to-date email addresses for me synced to my phone, so I see this as a good initial step.