One of the main reasons for using SCCM is to ensure consistency of the build. If there are 100 steps involved in making a "good" image, then the vast majority of these can be mandated and controlled by SCCM, thus ensuring that each step is done; consistently, accurately and in perfect order. Doing that many steps manually each time almost guarantees something will get missed or forgotten.

Another good reason is that steps ordered in SCCM are vastly easier to diagnose, interpret, adjust and review then scripts. This is a generalised statement, many of the people in these forums would argue with that reasoning. But I'm talking from a business ownership and complexity point of view, not from your average VMware engineers point of view. You may be awesome at scripts, but the guy replacing you two years from now might not be. Using scripts carries it's own set of risks that many businesses are not comfortable accepting.