Now ill have to make a raketeen and get an even weaker earlier game because of a WC51?
As if Volks alone were not weak enough...
Well, USF player makes himself weaker by getting WC51(much less shock value then M3 because of greater cost, delying important teching, removing 1 rifle squad you won't afford because of it and nothing like M3 flamers to actually exploit the advantage of WC51 garrison) instead of another rifle that will scale and be useful in late game as opposed to WC51, which most likely become scrap metal due to its bulky, slow cow nature.
Should you hinder yourself to counter a unit that just hindered your opponent? That is the question for you to answer.