Global hard-coded dependencies are bad not only for design-related matters.

global_dependency.rb

class MyEvilDependency
  def self.do_good
    "ok"
  end

  def self.save_the_world
    puts "I could destroy the world if I wanted"
  end
end

class InjectedDependency
  def do_good
    "ok"
  end

  def save_the_world
    puts "I would destroy the world but I can't"
  end
end

class NaiveClass
  def do_something
    MyEvilDependency.do_good
    MyEvilDependency.save_the_world
  end
end

class AnotherClass
  def initialize(other_obj)
    @other_obj = other_obj
  end

  def do_something
    @other_obj.do_good
    @other_obj.save_the_world
  end
end

describe NaiveClass do
  describe "#my_other_method" do
    it "trusts MyEvilDependency" do
      MyEvilDependency.should_receive(:do_good)

      NaiveClass.new.do_something
    end
  end
end

describe AnotherClass do
  describe "#do_something" do
    let(:obj) { double }

    it "does what I want" do
      obj.should_receive(:do_good)

      AnotherClass.new(obj).do_something
    end
  end
end

# $ rspec global_dependency.rb
# I could destroy the world if I wanted
# .F
#
# Failures:
#
#   1) AnotherClass#do_something does what I want
#      Failure/Error: @other_obj.save_the_world
#        Double received unexpected message :save_the_world with (no args)
#      # ./global_dependency.rb:35:in `do_something'
#      # ./global_dependency.rb:56:in `block (3 levels) in <top (required)>'
#
# Finished in 0.00103 seconds
# 2 examples, 1 failure
#
# Failed examples:
#
# rspec ./global_dependency.rb:53 # AnotherClass#do_something does what I want

Basically, you have no control over the side effects of a global dependency.