There are a range of advantages of using a self propelled gun for static operations. As demonstrated in VietNam and now Afghanistan. They include:
Fire through bombardment. When the enemy is shelling your own base you want to shoot back. With a towed gun the crew have to run the risk of being hit by the bombardment while they operate the gun.
Fire endurance. Most of the loading functions of an SP gun is automated making them much easier to operate for sustained bombardments than towed, manually loaded and trained guns.
Self sustainment. SP guns are integral units with everything they need to operate on board – just add fuel, ammo and humans. Towed guns need a lot more infrastructure to operate.
Rapid response. Turreted SP guns can quickly bring their gun to bear on any target while towed guns need to be manually swung around to face the threat bearing.
Mobility. The SP gun’s firing position can be located outside the base so as to minimize disruption to the base when firing. The gun simply races a few hundred meters and starts shooting. This was the SOP for the Dutch PzH2000s in TK. Also the gun can displace rapidly under protection with a unit of fire to another firing position to provide extended reach.
Bigger is Better. The longer barrel and larger chamber of the L52 155mm allows for more accurate conventional fires and/or longer range.
Direct Fire. Don’t have a tank on hand?
My issue with tracked SPA (my personal opinion) in A-Stan over light tubes, is deployability, moving a tracked asset to a rural site requires route clearence, engineers to strengthen bridges, infantry to protect the movement, IED mitigation (reactive & bar armour) & REME assets in case it breaks down. Compare this to lifting a light gun (105/155mm) under a Chinook.
The Vietnam comparison is irrelevant because VBIED & the IED threat was minimal plus SPA was moved as part of a much larger full-on battalion deployment, which were left largely untouched by the NVA. Whilst a light gun is unprotected in the FOB (no armour), they have proved battle winners and have not lost a single crew member to counter battery fire.
Air assets are vulnerable to weather conditions and have a limited weapons load. Light gun crews can sleep next to the gun-line and have 100+ rounds to hand, including HE, Phos & Smoke.
The Aussie desire for SPA mirrors NATO Northern Europe - heavy armour & SPA followed by armoured bridge layers and engineering units. All unnecessary for recent Aussie deployments in ET and A-Stan. Even if we had a GWIII, there is no way Gillard would deploy an armoured brigage, she would do what every other Aussie PM has done since Vietnam and deploy SFwith the odd training team in tow to demonstrate Aussie commitment to the Alliance. This view has been reinforced by her recent comments.
I'm trying not to be critcal, but the Aussie military is SF top heavy, so why not build capabilities around reinforcing this tool in the box, rather than trying to tick all the boxes with a tiny land army vs. population size.