My wife and I shared an Amazon account for a while (~10 years) because it was easier. If you live with someone and have joint finances, why bother with separate Amazon accounts?
Well for one thing, it makes keeping gifts secret more difficult. Seriously though, you could also share a Gmail account but why do that when there's no incremental fee (households can share a Prime account)? It just leaves you with a more confused data set and poor product recommendations.
Assuming that you're sincere in asking... neither my wife nor I care about the product recommendations and I don't see how "a confused data set" hurts me other than that (the data set's going to be confused by the fact that we frequently buy gifts for other people over Amazon anyway). Gifts are easy; you just tell the other person not to look at the orders because you bought him or her a gift.
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.
So I do share my Amazon account with my partner, but only so we can have a single Kindle library. Other than that, it seems like a disaster... obviously it's personal preference, but it's a lot easier to deal with security and data models when you're using them the way they were built to be used (1 person to 1 account.)
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.
