The most important things we produce in HR aren't easily measurable. Take Gary's triangle of engagement needs - things at the botttom like attendance, compliance, obedience are easy to measure. Things at the top like passion, pride, creativity, love aren't. The result of trying to monetise HR outcomes is that we end of focusing on the wrong ones.
Good HR is one of the key enablers to adaptable organisations and tying our metrics to financial outcomes cleary reduces our organisations' ability to adapt.
It also anchors our organisations in BAU mode. Much more important that linking HR to the way we currently run our businesses is helping businesses understand why things need to be managed differently.
We can manage what we can't measure and we need to build tolerance for leading in areas of increased ambiguity, including in HR.