Almost all Excel users are not programmers, you're definitely right. If by clerical workers you mean people who are just entering data into the sheet or reading an output then, yes, they don't need to understand what the sheet is doing.
However there are loads of people who build there own sheets: analysts, consultants, managers, marketing, accountants and yes engineers. These are the people who spend the most time using excel. Excel let's you build a very complex model formula by formula. It's often a model more complex than you might be able to map out initially. Whilst they are not programmers they are still smart enough to understand the sheets they've built. The issue is Excel right now is so opaque about what its doing and where the numbers come from. It makes this task much harder than it needs to be.
However there are loads of people who build there own sheets: analysts, consultants, managers, marketing, accountants and yes engineers. These are the people who spend the most time using excel. Excel let's you build a very complex model formula by formula. It's often a model more complex than you might be able to map out initially. Whilst they are not programmers they are still smart enough to understand the sheets they've built. The issue is Excel right now is so opaque about what its doing and where the numbers come from. It makes this task much harder than it needs to be.