Why Artillery Reigned
There was a time when people thought Recons were useless outside of Fog of War matches. This meant that the first Directs to hit the field were Tanks. The problem with Tanks is that, as a unit trying to do crowd control on the far more numerous Infantry, they're rather slow and kind of weak for costing 7k.
Enter the Artillery. The Artillery could control ground while protected by an Infantry screen. This can only be taken down by getting mobbed, which requires mobbing units to be closer than their maximum attack distance. (Otherwise they won't all be in range.) This makes it pretty easy for a Tank or a Copter hiding near the Artillery to run out and get the first strike. Since the Artillery is cheaper than the Tank, the walling player is being more cost-effective with her units. Naturally, this presented quite a problem: You have a poor crowd control unit trying to pull double duty on crowd control and offense, while being vulnerable to attack itself. As such, you begin to see players approaching the game much more conservatively, like an elaborate game of Chess.
Recons as Crowd Control
In 2010, Recons finally saw use in a capacity that had mostly been missing from the Advance Wars metagame: dedicated crowd control. Recons are the cheapest units besides Soldiers, are effective against Soldier and Indirects, and have very good movement. Two or three Recons can slow or halt Infantry movement on a large section of the map, and combined with most SCOPs have a very easy time mobbing walls. They're also just as efficient as Infantry at filling up your CO Power bar.
Recons don't just counter Artillery walls: they make it impossible to form one. Recons are cheaper, fill the bar more efficiently (both give you one star upon death, but again, Recons are cheaper), and more cost-effective. Artillery can attack units at most tech levels, but Recons project a far greater zone of control. Artillery can't slow or prevent captures; Recons can. Artillery need Tank support to be effective; so do Recons, with the benefit of Recons still being able to move around and attack. Overall, there's just no reason why you would ever build an Artillery over a Recon these days.
Just what are Artillery good for, anyway?
Artillery are used for clog control now. In a metagame that no longer tries to use them to project a zone of control, it's easy to get to a point where so many Directs are hitting the field that not all of them can attack. An Artillery or two can open up some space without hogging any themselves. They have to be quite a bit closer to the front in order to serve this role, so some folks might be tempted to get a Rocket instead. Unless you're Hachi*, Rockets are too expensive to be justified, so just don't do it. Losing a couple Artillery due to getting exposed at the front line is preferable to spending more than twice as much on a unit with only 5-15% more firepower.
*If you are Hachi, you have better things to be buying anyway.
If Recons do crowd control, what are the Tanks for?
Tanks are Recon- and AA-hunters. If Artillery are on the field, then they are also Artillery-hunters. Tanks can assist with crowd control, but the player should bear in mind that this is not their primary purpose, nor should they try to make it so.
Tanks are usually your bunker-busters during SCOP, provided you are using a CO that permits this. The rest of the time they should be focused on their primary targets and protected by an Infantry screen when possible. They usually should not take part in grinding out a SCOP; Infantry and Recons feed the bar much more efficiently and should be used for that purpose. Tanks should never try to attack a tech unit without the assistance of a COP or SCOP boosting firepower and/or defense.