Testing (WPS/PQR/WPQR) requirements varies greatly. Many CWI's are not Deputy or Special Building inspectors. Many CWI's work in the aerospace or petrochemical industries, they would not know the requirements that the Building/Code officials specify without going to sit another exam.
By the way, many jurisdictions require their own exam in addition to a ICBO special inspector examination/certification. Also, a CWI is not enough to perform Deputy/Special inspection, the Building Officials are now starting to look at the CWI program in addition to the other testing required to perform weld performance or procedure qualification.
The problem most have is that they assume if a CWI is specified in the contracts, this is the best choice as far as structural steel and welding is concerned. WRONG.
For many projects here in CA, they require a Deputy inspector to have a CWI in addition to the other certifications or licenses. This may soon become required by the city and county of Los Angeles.
A CWI does not have the knowledge (without training) about high strength bolting, testing for these connections, erection tolerances, building code requirements, and local jurisdiction requirements. Did I forget plan reading? As a building inspector, it usually requires bringing the D1.1, D1.3, D1.4, AISC ASD, the UBC or IBC, and finally any local jurisdiction codes. CWI's do not what some of these references are.
An ICBO inspector typically does not know the welding material as well as "most" CWI's such as control of consumables, reviewing welding related documents, etc. Imagine having an ICBO inspector looking at a cat-cracker or knockout drum unit in a petrochemical environment...
These are two very different exams for two very different inspection types. It has been proposed in the past to grandfather a CWI into the ICBO certification - WRONG AGAIN. This will only further undermine the building inspection trade. Many Building Officials and Engineers complain about the quality of inspectors, but do not know why the CWI did not verify the conditions of the faying surfaces of that slip critical SMRF connection. Conversely, the ICBO inspector they requested did not know about the electrode storage requirements or what to do with the WPS he/she was given to review for the project.
I'm not saying a CWI cannot perform as a building inspector, a qualifier of welds (performance or procedure) or vice versa. I am saying both of these certifications have their place. Ultimately, the individual determines how well an inspection proceeds.