Who are we kidding... is there really a debate here? MIM, metal injection molded parts have allowed production firearms manufacturers to produce guns at a much more affordable price in a highly competitive market. The firearms industry is extremely price driven, and as a result, material and process costs are a huge factor in overall manufacturing decisions. Castings and MIM parts have been used in every industry to keep costs down, but at what cost? Just like anything else, I do not believe that all MIM is created equal, and any gunsmith can tell you that they have seen their fair share of MIM failures in this industry, but the old adage remains, you get what you pay for.
The MIM in the firearms industry is less dense, more porous, has less ductility and less tensile strength than forged and billet counterparts. Other industries have developed MIM parts that have matched ductility and tensile strength in many applications, but the firearms industry is largely behind in this area. The use of MIM is mainly used in guns which are relatively inexpensive and marketed to people who do not use them much... a hunting trip here and there or the casual trips to the gun range, for these people, the low cost MIM parts work just fine. Competitive shooters shoot tens of thousands of rounds per year and shoot on a weekly basis. They require guns to be able to handle the abuse or high round counts combined with lower relative maintenance. This is where MIM gets a bad name. When used in competitions guns, MIM parts WILL likely fail. The custom market was founded on taking production guns to a competition level in terms of both accuracy and reliability. Custom guns needed to be able handle the abuse, do it reliably, while maintaining accuracy.
So the question... is there really a debate? The answer to the question is no. The use of MIM parts are perfectly acceptable in low priced, low use guns. The use of MIM is not an acceptable material for parts used in custom/competition guns sold at a premium. As a consumer you want to not only get what you pay for, but also get the reliability you require out of your premium priced custom guns. If your custom smith or favorite semi-custom company is using cast or MIM parts, ask them why? If the answer to that question is because the parts are good enough, it should make you wonder what else was JUST good enough on your premium price custom gun. If you are paying a premium price, demand premium parts and execution. If you are paying a budget price, expect budget materials and execution. It is just basic economics, nothing really to debate.