What point is there in using an everyday non-admin account on OS X?
Security is best implemented as a Multi Layered, Multi Vectored Strategy.
The Principle of Least Privilege (POLP) is just another cog in the machine that keeps your computer secure.
Everything that you listed there are all good but none of that will prevent someone from taking over your computer with an exploit such as the Dropped Drive Hack.

How does a firewall prevent a user from inserting a USB with an remote control exploit embedded on the drive?

How does SIP prevent a keylogger from capturing your keystrokes?

How does having SIP even matter when it can be easily disabled by the Admin?

How do you prevent unauthorized/illegally licensed software from being installed?  A restricted user account will ensure that users who shouldn't be installing software aren't installing software.


Your last line of defence is using an account that is not an Admin account so that you can mitigate the threat by putting up another layer of security (user authentication) when a piece of malware tries to install itself.
I have been saying this for what seems eons now:
&quot;Security&quot; is not a product you buy or a switch you flip on; it's a practice, it's a mindset, to leverage all the tools you can to minimize your risk.