Yes but investing manpower early into a subpar AT gun can hurt quite a bit if you don't manage to kill the WC51 and the opponent uses the refit ability on the WC51 to get his resources back. Basically means you've wasted manpower and sacrificed map control.
I do agree on that point, but mostly put it in a building will mean that he can't use a big portion of the map, which means you can consolidate your forces and set out together to cap/give a push.