I think the author misses the point of the `let` keyword. It's not meant as the equivalent of declaring constants in Objective-C; they're main purpose isn't to be global and static like constants.
To shed some light on the practice of using `let` vs. `var`, an excerpt from the book by Apple:
“If a stored value in your code is not going to change, always declare it as a constant with the let keyword. Use variables only for storing values that need to be able to change.”