Why are rings unique equipped?

[Zyrusticae]

I see people do this a lot and I've always wondered if the secondaries on the "off" ring outweigh losing so much of your primary stat.

They don't. Det and Crit are worth roughly 1/3 of your primary stat. In other words, you would need 3x what your primary stat would be to make it worth it. This never happens, naturally.