Their main value is their speed to run around and capture empty provs real fast.

They are good for taking enemy regions, and early on, they are one of the better units against infantry. They can also be used well as back line defense against Infantry.

Like DxC said, their main role is to capture unoccupied territory. This is only really effective against players to use after a breach has been made in their front. I once captured 90% of Russia (a player, not an AI) with two of these. Not as effective vs. AI because they leave their units spread all over hell. I almost never upgrade or purchase this unit. The few you get at the beginning should be able to survive the whole game if you're using them correctly. Apart from the opening days where you're rushing to establish borders, these should never be used in actual combat and should usually be separate from the main force, unless you keep them together only to split them off after a breakthrough. Fully upgraded, I believe they are the fasted land unit in the game, but their combat stats suck so much that you hardly ever upgrade them. Light tanks quickly fill in this role, as they are worth upgrading, and eventually become faster than a lvl 1 armored car. Definitely a specialist unit, and not worth building a lot of them, but it is advisable to keep a couple alive so that they can fill this role.

They are very useful to capture unguarded provinces. If you have enough, they can take a lightly armed province. Also, they are the best way to combat infantry early in the game. I think they are worth it in the beginning, but when you can use tanks, use them as they are much more powerful. They can survive for a while, but you should only keep 2 or 3 later in the game in the event of a breakthrough.

btw, as Tygus mentioned, I don't waste research time on ACs. Light tanks quickly become faster and ACs completely irrelevant. A lvl 2 LT is the same speed as a lvl 1 AC and a lvl 3 LT faster. One benefit though is that build time for ACs is fast. If you are conquering empty provs and need more "run around" units you can quickly build ACs in newly conquered cities. They take slightly longer than militia and AA but sometimes worth it since even lvl1 AC is pretty fast.

Mostly I use ACs to defend against rebellions. They don't move as fast as a upgraded light tank (the bulk of many players' forces) but they're strong enough and are still a lot faster than infantry. They won't be strong enough to fight against a well-equipped enemy, but they'll be good for securing areas after the fight's over.

Also, I usually don't upgrade this unit. I've only done it twice, when I upgraded it to level 2. I've never gone beyond that.

I think armored cars should be renamed reconnaissance regiment. They should offer even greater line of sight and be just as fast as motorised infantry. Consider them a mix of armored cars and motorised/motorcycle infantry. They should have 150% the soft attack of infantry and basically be used as recon. 25% penalty in mountains and cities as they are half soft themselves.

I may be one of the few who use ACs. I like their LOS and speed I like to attach them to artillery generally. Then once the arty takes down the enemy rush in with the AC.

I suggest increasing the distance they see stuff and their speed. It's not realistic that ACs have the same speed as LTs.