Multi-tasking by it's very nature severely limits high quality results. It is the illusion of efficiency.
All it really guarantees is that you'll 'touch' it. So you have multiples of 'touching' things, issues, projects. This is not the same as dealing efficiently with multiple details in a high quality way. How can it?
That definition is the illusion. It is unattainable, demoralizing in both the long term and short health of the person attempting it, and defeating the purpose and goal of high quality results.
Is your goal to produce high quality results? Do you want projects handled with a high level of attention given to detail?
To use a golf axiom to illustrate my point here. Drive for show, putt for dough. Multi-tasking is the act of driving for show. Tiger Woods putts for the dough. His singleness of purpose is to get to that green so he can putt, for the dough. It doesn't mean he doesn't do what needs to be done to hone the skills necessary to get him to the green. But his craft in completing his mission, closing the sale, so to speak, is to get to that green, so he can putt.
So where does that take us with Multi-tasking? It is true that we need to be able to 'manage' multiples. However, it is unrealistic to set your sites on bringing multiples to their conclusion, simultaneously. Closing the sale takes attention, focused attention. If you are not allowed to give this proper attention to the details of quality, you will not produce quality results. We should not be surprised by this phenomenon.
Important people and projects in our lives deserve dedicated attention. Giving your full attention to a child cannot be substitued with anything. Nothing else measures up to a child like having your undivided attention. Roll that around in your mind for a while. They will go to great lengths to get your undivided attention. Explains a lot about the behavior we witness in children, doesn't it?
If an employer expects their assistant to be an expert multi-tasker. That employer should not be surprised that there will occasionally be areas of weakness witnessed in the results. You sacrifice quality for the quantity folks. Plain and simple. Think about what you're asking for and quit beating up your employees because they don't 'measure' up. Your definition is faulty. That's what I'm saying.