Question

If creating a class variable is often dangerous and unpredictable why do we need them?
If solution is just to use class instance variable with the class level accessors:

class Foo
  @variable = :something

  def self.getvariable
    @variable
  end

  def self.setvariable(value)
    @variable = value
  end
end

Then why do we need class variables???

Answer 1

Class variables have their use on occasion, but I agree that using the eigenclass is frequently more useful:

class Foo
  @bar = 'bar'
  class << self
    attr_accessor :bar
  end
end

puts Foo.bar         # bar
puts Foo.bar = 'baz' # baz

The above is safe with inheritance, because it sets a variable in the Foo constant, rather than a class variable.

Foo.new.instance_eval { puts @@bar } # error