The argument works just as well with "the person who is bad at IQ tests gets repeatedly hired for lower-paying jobs" as it does with "the person who is bad at IQ tests gets repeatedly not hired at all". Back when you were permitted to discriminate in hiring based on race, black people didn't have absolutely no jobs--they were just hired for jobs that were generally worse. (And people didn't notice the good blacks should be in the office and promote them at a higher rate to make up for it, either. Rather. they allowed their bias to affect their assessment of how good blacks were at their jobs.)

Edit: There's also a problem that's related to the first but where accuracy isn't involved. Imagine that IQ tests were always accurate for job purposes. 100% of the time; if one person has a higher IQ than another, he has higher performance.

Employers would then start hiring people from the high IQs down. In a limited job market, employers would stop hiring before they reached the bottom. Someone could find himself having 95% of the productivity of someone with a higher IQ score but hired 0% of the time. Again, it's bad to have people who are hired 0% of the time.

You could solve that by introducing noise into the IQ scores, but of course that is equivalent to not allowing IQ testing and forcing employers to use noisy measures of IQ.

(You could also solve that by allowing employers to hire one person at X% of the salary of another, but employers tend not to do that even for the measures they are allowed to use now.)