The cost of quality control is always an issue with a fabricator or a to the purchaser. Anything that can reduce cost and reduce the manufacturing time is worth considering. Generally, visual examination is the most cost effective method used to ensure the product meets the requirements of the contract documents, i.e., purchase order, code, project specifications, and drawings. As mentioned in a couple of posts, it is usually efficient and cost effective to perform the VT prior to additional NDE. However, what I've personally encountered has been contractors that attempted to use PT, MT, RT, or UT in lieu of VT. The opinion of the contractor's inspectors was that other nondestructive test methods replaced the need to perform VT.
RT, UT, PT, or MT cannot replace ongoing VT. Each test method, including VT, has limitations on what types of discontinuities can be located and properly assessed. I believe the various code bodies recognize the limitations of NDT and recognize the value of VT as being the most economical and timely method of locating and identifying deficiencies when they occur so that corrective measures can be to eliminate the unacceptable product from the production stream or to repair the deficiencies as they occur. this reduces cost by eliminating investing time and labor into a part or component that will not be acceptable. True, the discontinuity may be found (I did not say, "will be found")by another NDE method, but the cost of repair will be greater and the labor and time invested since the defect was introduced is lost, thus driving up cost.
As an example: Laminations along cut edges can often be detected visually. If the visual examination is not performed and the cut edge with the lamination is incorporated into the welded joint, it may be located and identified once the joint is welded if UT is specified and the technician uses compression waves (straight beam) to interrogate the joint along the edges of the welded joint. However, the cost of repair includes the time and labor of the original weld, removing the existing weld and adjacent base metal, and rewelding the joint. It RT is specified, the lamination will not be detected and the defective part will be shipped where it may fail while in service. An in-service failure can cost the company that produced the defective part, well, who knows how much the in service failure will cost in terms of injuries and associated damage.
A program of thorough on-going visual examination at all stages of the manufacturing process is much less expensive that performing NDE at the end of production when one considers the NDT method selected has the potential to miss critical subsurface defects. I used RT as an example, but the danger of a manufacturer or contractor substituting a different NDE method in place of VT is mitigated if the code requires VT be completed prior to the implementation of additional NTE methods. I believe it is the intent of AWS D1.1 that VT must be performed before other NDE methods are performed. Any deviation from the sequence specified would have to be approved by the Owner's Engineer. It is not a decision the contractor can make unilaterially.
Best regards - Al