The incremental cost is in keeping shipping addresses and credit card info up to date in two accounts vs. one and in having to remember which account was used to order something if we need to look up order info.
The only advantage in having separate accounts is when it comes to commenting and reviewing; I'm picky about only having reviews that I've written listed under my name, so we got separate accounts when she wanted to review some purchases. Many people (and probably most nontechnical people) do not care at all about the "benefits" you mentioned. Plenty of couples share email accounts too. I've even seen shared Facebook accounts.
From your description, it sounds like you really have two accounts, but you just do all your purchasing from one.
Sharing a free email account just seems like an exercise in frustration. The whole usage model for email - inbox, notifications, read/unread, filing, deleting - is designed on the presumption that the messages are being consumed by a single individual. I've always viewed sharing an email account as something that old folks do because it maps to their model of a physical mailbox, or because they're too feeble to maintain / login to multiple accounts.
Having worked for Amazon, I suspect you're not getting the benefit of a lot of their recommendation technology if you're sharing an account. It's not just about purchases - it's about every page in your clickstream and how that's used to find products you'll like. We actually had projects in some services dedicated to reducing sharing of accounts, partially for that reason.