Suppressing fire is going to need more than 1 tube if you are trying to keep enemy mechs, armor & BA from taking firing positions. While I generally support pre-spotted hexes, b/c I know what IRL accuracy (and math) is like, I do not support a attacker getting to select a fortified position hex as a pre-spotted location . Additionally, you might want to lay down smoke to block fire from the side positions while your forces advance on the section of a line you are pounding with HE . . . more than one tube also allows you to follow the shoot & scoot doctrine to avoid counter-battery or airstrikes against your firing units while keeping up the pressure, especially if its Thumper field guns.
Oh yeah, and if it was not mentioned, field guns are absolutely a reason to use Sniper over A4 for the ammo reason, since a field gun only gets a single ton of ammo.