TL;DR: It's hard to defend an IQ test against a discrimination lawsuit.
Unless there's an explicit business need to concretely identify someone's IQ, a business is at risk of being called out as using the IQ test to discriminate against some protected group, since that protected group is potentially disadvantaged while taking the IQ test by any one of dozens of (legitimate) reasons.
http://en.wikipedia.org/wiki/Griggs_v._Duke_Power_Co
TL;DR: It's hard to defend an IQ test against a discrimination lawsuit.
Unless there's an explicit business need to concretely identify someone's IQ, a business is at risk of being called out as using the IQ test to discriminate against some protected group, since that protected group is potentially disadvantaged while taking the IQ test by any one of dozens of (legitimate) reasons.