> It's hard to know whether you've followed the standard correctly if you can't verify the output.
Being able to verify certainly makes it easier, but a standard like "Ensure all images have alt-text" does not require a screen reader to verify.
And the thing with standards is that often no existing product (screen readers in this case) will fully comply with the standard - so if a given screen reader fails on your site, it may well not be your site's problem. This was (and perhaps still is) the case with Web standards in general. In the old days, there was usually not a single web browser that was fully compliant.
Your reasoning for why one shouldn't want to test with screen readers seems more like reasoning one should want to. The more divergence from standards the more you want to validate it works as desired not the other way around.
You still need some sort of verification, even for something as simple as ensuring that images have alt-text. Maybe you missed some images that are dynamically generated in a way you forgot about, or have a typo in the tag.
If screen readers aren't compliant to the standard, then you need to test against as many screen readers as possible to ensure that your site is accessible. Users want a site that works, not a site that complies to an unimplemented standard.
Being able to verify certainly makes it easier, but a standard like "Ensure all images have alt-text" does not require a screen reader to verify.
And the thing with standards is that often no existing product (screen readers in this case) will fully comply with the standard - so if a given screen reader fails on your site, it may well not be your site's problem. This was (and perhaps still is) the case with Web standards in general. In the old days, there was usually not a single web browser that was fully compliant.