Big Question: are public inquiries a waste of time?

Propsect Magazine asks if public inquiries fit for purpose, in the light of delays to the Chilcot and child sex abuse inquiries. Article quotes Peter Riddell, IfG Director, as an expert commentator on public inquiries:

Public inquiries can be useful provided their terms of reference and procedures are clearly defined from the start. The problem with Chilcot is that its remit was too wide from the start. It substantially  underestimated the resources required and has appeared to be  submerged by a sprawling mass of evidence and assertions.

The same risk applies to the prospective child sex abuse inquiry. Problems also  occur when inquiries are set up at the same time as the police are conducting investigations (which constrained  the Leveson inquiry into the press and caused the abandonment of the Detainee inquiry before it was launched), or when secret and confidential material are involved. The Soham inquiry was successful because it was set up after the criminal cases, was tightly focused on a limited number of issues and closely engaged those affected.