That is a fine method when you are dealing with one set of methods for the shell that is used over and over - and that is what I use in such cases. However, you misunderstand the purpose of that simple example - sometimes you need to do a simple task or multiple simple tasks. In one of my projects where I must run around 120 shell processes, it would become rather bloated in the code to create 120 Shell object instances because the Execute and processing code is dramatically different for each.
And as you state, your comment, your method is correct in your humble opinion. Remember, if you ask 50 coders how to accomplish a task, you’ll get 65 different answers - all of them correct in their own mechanism and purpose (Wayne’s comment about AddHandler being one of the different answers 
)
I will just add that if the Completed event is not firing for you, I would file a bug report on your version of Xojo. I can run your code and the Completed event fires properly here under Xojo 15r2.2.