Stock USF M5HT cost 310/50, for 90 manpower more you've got the ability to randomly rape your opponent troops and base from behind your T3. Not to mention that on certain maps You can place quite safely behind you T3 to cover your fuel letting you harrassing his side of the map.
Anyway Howitzer for OKW was a stupid idea since it was first mentioned.
This is comparing apples to oranges. Also OP stated this happens even when he is ahead, thus this investment of Axis is much more severe, and I believe OP just has a problem what to do against Howitzers.
Not only is it hard in such a situation for Axis to get fuel back but also getting the Mp back. A howitzer takes 13 popcap.