How much more reliable would hiring and promotion decisions be if assessment tools were used to supplement personal interviews, intuition and past performance?
I am not persuaded that there is a reliable quantitative answer to this question.
I usually recommend and assist with a three-step process:
1) Quality interviews (regrettably, a rare-ish commodity)
2) Quality reference checks (same comment)
3) A psychological profile - I use Insights
All three elements provide lots of useful information.
The interviews are the most important, references next, and the Insights is used as a back-up. If all three elements are consistent, we will likely proceed with the hire. If there are inconsistencies, we will likely look further, but our antenna go up immediately.
Mainly, we are interested in the "fit". Technical abilities can be assessed quite reliably, but the "fit" is more difficult, and at least as important.
I can't put a number on the increase in hiring quality that comes from this approach, but I know it is significant, and a very good investment in terms of time and money. The cost of a poor hire is very high, and avoiding the need to undo a poor decision is very cost effective.