Haven't tested anything, so this is pure theory-crafting based off the stats, but I don't see that it really offers you anything worthwhile for a defensive division. (Note that all of this is defense-specific - they're not exactly good on the attack, but they do at least offer some breakthrough).
Armoured cars have trash defense, so you can't make a primarily car-based division for a garrison or it will just melt. If you're thinking about splashing in one or two battalions in an infantry division, they don't look like they provide enough armour or hardness to really be worth it. 1940 cars are equivalent to 1934 light tanks at only 10 armour, so one isn't going to be enough to get the unpierced bonus even against pure infantry - let alone a single support anti-tank or anti-air company. If I've calculated right, two battalions in a small division (<10 battalions/companies total) would barely scrape over the mark - until you take a few losses or the enemy brings that single anti-tank company, anyway.
If you get 1942 cars and you expect to be fighting infantry/artillery without any anti-tank support at all and the infantry hasn't got 1942 guns of their own, then the cars should actually meaningfully contribute to your combat effectiveness. That's a lot of hoops to jump through, though (and you'd have to actually commit research time to it as well - it's not just 'grab the first car tech whenever you get around to it').