I would argue that the premise of the question makes a wrong assumption:
There is an enormous amount of people that use computers set up to perform a single task. Behind the scenes, they're generic systems running a full OS and having all capability, but the machine has been specialized for some tasks.
For example:
* Cashiers use machines which are commonly running Windows. 81
They do not have access to the computer's OS: the machine boots up in the only interface they'll use during the day and they do not need all the complexity.
* In supermarkets, devices scanning for prices and even electronic scales are sometimes running a full OS, but the user doesn't see it.
* Your TV set-top box might be running Linux and could be doing a lot of things, but it is abstracted from you and you just see the interface designed for you.
* In many offices, people have access to a single piece of software, to perform their tasks, and never see the OS.
In the professional world, you have a lot of situation where the complexities of the system are hidden from the user because they are locked in a single software (cashiers are the most obvious example).
When the work is less repetitive, people will have access to the computer OS because they will need a variety of software to perform their tasks; in this case, they'll need some basic computer skills.
For economical reasons, it's better to get standard PCs with one of the common OS and then specialize as much as you need from there. This is easier to setup, maintain, update, etc than a custom solution that might be locked in by a vendor.
Some companies, like http://www.citrix.com for example, are providing solution that turns a full flexible PC (read cheap and common computer) into a specialized device that will run specialized apps so that employees have all technical problems abstracted from them.
As I was saying at the beginning: I believe the premise of the question is flawed in the sense that there are millions of flexible computers set up to let employees just use the features they need and nothing else. In that context, all complexity is abstracted and usability is directly linked to the designed of the apps they'll be using.
When it comes to personal computing, it's a different story since every user has his/her own use cases. 81