I mean, I've used uncertified oil before, when it's from a company I trust. With Valvoline's VR-1, if they say it's 10w-30 and with, say, SJ additive levels, I believe them. I'd trust Royal Purple, Amsoil, and Red Line uncertified oils (all within reason of course). But, if it's something I've never heard of with no API Donut, that's more of a problem.
The API Donut and other certification markings are there to make sure that people know things are certified, particularly those that know little about oil. It also helps police the market some. Most BITOGers know what SN/GF-5 is with or without the Donut and Starburst. OEM manuals and oil company literature do have to depend on certification and the marks for public education. Plus, a minimum certification is always nice.
Note that some companies have earned enough trust over the years that their uncertified products are trusted to be what they claim. That doesn't apply to a lot of companies out there.