The issue with tobins-q is that the economy is less capital-intensive than it used to be. So valuing a business by it's assets is harder to do, unless you quantify the brain power that now makes up the vast majority of many companies resources.
For example, the value of facebook does not lie in its assets, it's in their online user base and ability to create great user experiences.
On the other hand, when US Steel was one of the largest companies in the world, it was relatively easy to sum up the value of their assets and thus, tangible worth.
