The real reason is to reason to have multiple accounts is to avoid commercial scum. And not ever, ever to use email providers like google and yahoo and microsoft, etc. I use tutanota. Proton mail is fine too. I have multiple accounts with tutamail. While google or yahoo can get emails that I send to people who use google accounts, they don't get everything. They don't get every single email to shuffle through.
Don't worry about NSA. Worry about google, yahoo, and all the companies that they sell the information to. That's the real issue, if you think about it.
And the NSA is not some magic eye of Sauron. You can dodge their gaze. Yes you need 'ridiculous' amounts of effort, but that's the price one pays for being anonymous. You need to make surveillance expensive, and this is done through good opsec combined with properly implemented cryptography (no side channel attacks possible). Snowden changed the game here. Many have gone dark and the NSA is now compromised as a result.