Isolating resources is definitely a valid and widely used method of controlling security threats.
I can think of many scenarios where these benefits of isolation would apply to email addresses, including those you mentioned.
Are there any down sides to doing this?
I don't think there's any real downsides, but I can think of a number of reasons it would be difficult for most people to derive the full security benefit of isolating their email addresses.
Firstly, you need to minimise dependencies between accounts as these weaken the isolation. For example, you'd have to avoid:
- Using one account as a recovery email for another
- Sharing passwords between accounts
- Using the same domain across accounts (especially if it's your own domain)
- Sharing an actual inbox (ie. destination for all your mail)
This creates many inconveniences:
- It may take a lot of effort to setup many mail accounts, especially on email services you don't control (eg. gmail)
- Having to check many inboxes
- Having to maintain many sets of credentials
- Having to constantly make decisions about which address to use in order to minimise risk
One common technique for people who own a domain is to have a catch-all rule that goes to a single inbox. That way they can invent an arbitrary email address every time they need one (eg. firstname.lastname@example.org) and if they get spam from a particular service they can easily block the unique address they used. This will help with the spam scenario you mentioned, but not if your account credentials are compromised seeing as all addresses will share an inbox and password.