The Howitzer turret can be decent on defense, providing it targets the correct unit such as Infantries and sometimes Light Vehicles. However, it can miss and if the ground units get into its falloff range, it will be useless. So howtizers suck!!! people use it as a shield.
Light Vehicles can dodge its shells and get into its falloff range easily, while Tank-role units like the Rhino can block its splash radius when the Howitzer targets them first, reducing its effectiveness. High HP units are also capable of mostly shrugging off these attacks, as it's damage is rather low.