It's not really cheaper than the Kubel at all. It's 200 + 45 munitions for the ability to defend itself, I don't know anyone who doesn't get the upgrade. The arty barrage isn't even available until Major and without it there would be no reason to build one late game.
200 MP is a small price to pay for the package and advantages compared to the kubel(210 mp if i am not mistaken), even if it takes more skill and time to get it to vet 3. The upgrade is there to delay the .50 HMG that can easily drop models of infantry as it is a single entity with great DPS.
Arty barrage is easily more powerfull than the one from major, and it has a decent range too(though you still need to cover it obv). I'd say the risk/reward gives a bit too much reward.
Without arty barrage you still have mark target, that provides decent debuffs against axis armor and is available from the start.
At the very least the crew needs to go. The kubel does less and costs 2.5 seconds of manpower less(40) and cannot self repair w/0 doctrines or vet 3. This is aside from the ridiculousness that a visible crew of 2 creates a 4- man squad when you press T.