Lets say you're cutting webbing. You start with the serrated, cutting well, and end on the plain, cutting poorly. A fully serrated blade would have finished the cut. Crippled.
You're attempting a push cut where you want a clean resulting edge. As the blade is short, you wander into the serrations. At the transition the cut is no longer clean, nor do the serrations make a clean cut if you continue to wander. Crippled.
Using just one section of the blade for optimal cutting of whatever style is more difficult because of the design. Crippled.
Fine cutting is often done at the base of the knife edge for extreme control. Serrations mess that up unless you choke up on the sharp serrated "ricasso". Dangerous design. Crippled.
Sure, it can work OK for many other examples, but each of those examples would work just as well with a fully serrated or fully plain knife, depending on the situation. Serrations have their uses. Plains have their uses. Combos do too, but are more limited over all; crippled.
Phil