People consistently talk trash about MIM in online forums, however, I have never, ever seen a MIM part fail in real life. I'd like to see some information on gun numbers vs. failure rates from multiple sources.
The MIM process is capable of producing parts that have machine like tolerances, +/- .002", without the expense of the machining process. This makes for high quality, high tolerance parts at a low price.
When sintered, the plastic binders burn out of the metal. Casting is far less reliable and is still used widely. Forging can even produce defects in steel, both mild and stainless. If MIM isn't used, you have to machine parts from either forged of cast metals.
I had a Kimber Pro 10 II that I dearly loved. It was synthetic frame 1911, 10 round capacity, 4 inch barrel that went for $599 brand new. It is their cheapest model, but what a GREAT gun. As soon as I shot it, I instantly knew why there are so many 1911 fans out there.