Global variables are not bad. They're not evil. They're just incredibly, incredibly powerful. Which is why you shouldn't use them.
Global variables are global- they can be accessed and modified anywhere in the code. A single global variable has the potential to affect all of your classes, all of your functions, all of the classes and functions of every single library or dependency you load into your project, and all of the classes and functions of every single project which loads your project as a dependency, as well as the projects that load those projects, and so and and so forth, for ever and always, for the rest of time.
The second people start feeling comfortable using global variables, the namespace gets insanely cluttered and we get conflicts left and right and the stability of the programming language itself is threatened. Which is why the use of global variables is emphatically and repeatedly discouraged.
But global variables are not bad. They're like the highway lanes labeled "for emergency vehicles only," or like those fire-axes behind glass labeled "break glass in case of emergency."
It's entirely possible that at some point, in the distant future, you will have an incredibly unusual situation which merits the use of a single global variable. But that day is not today. And it is probably not tomorrow, or a month from now, or a year from now. Daily life, daily code- it just doesn't call for the unbridled power of a global variable.
$stdout is a great example of why global variables are sometimes important. $stdout is the default stream in ruby- the one where things will print if no other stream is specified. $stdout should be accessible from every class and every function in every library because it acts like a giant funnel, shoveling all output to a single location. The whole world knows and agrees that $stdout exists in ruby, and its uses are well-documented, so its power is well-managed.
This isn't to be confused with STDOUT which is a constant representing the actual pipe which sets up a stream between ruby and its parent program (usually a terminal). $stdout = STDOUT by default, but $stdout can be changed to anything. If you want your program to print to a file, you can change $stdout to a file stream.
I don't think this name choice is confusing for a seasoned rubyist. A variable is designed to be modified and a constant is designed to be constant. The difference between $stdout and STDOUT is that the former can be modified to change the standard output location of your program, and the latter is a constant, always pointing to the stdout stream. The capitalization makes a world of difference and conveys very different meanings.
As for why global constants are uninitialized and global variables are nil, that actually has nothing to do with globals. Ruby automatically initializes all variables as nil. You can easily see this with instance variables such as @foo or @@foo. In almost every situation, an undefined local variable will throw a NameError because ruby cannot tell whether it is a variable or a method. But in strange situations, they too are initialized as nil:
puts foo # => NameError: undefined local variable or method 'foo'
foo = 42 if false
puts foo # => nil
puts bar # => NameError
bar = bar
puts bar # => nil
It was a conscious design choice in Ruby not to automatically initialize constants. Because a constant is, by definition, something which is initialized once and then never changed, it would break the definition for a constant to be nil at first and then a different value later in the code.
I should also mention that global constants are considered acceptable, even among people who tout global variables as bad. The difference is that constants can only be assigned once and generally throw a warning or error if they get assigned again. This protects programmers from situations where conflicting global constants might cause problems.
