Why I don't like method missing

gistfile1.rb

require 'benchmark'

class A
  def m
    @foo = 1
  end
end

class B
  def method_missing(meth, *args, &block)
    if meth == 'm'
      @foo = 1
    end
  end
end

class C
  define_method :m do
    @foo = 1
  end
end

a = A.new
b = B.new
c = C.new

p Benchmark.measure { 10_000_000.times { a.m } }

p Benchmark.measure { 10_000_000.times { b.m } }

p Benchmark.measure { 10_000_000.times { c.m } }

# Output : 
  1.300000   0.000000   1.300000 (  1.306174)

  4.170000   0.010000   4.180000 (  4.179858)

  2.030000   0.000000   2.030000 (  2.042580)