Take my recent game with Kevin. He was able to set up each flank with a unit of spearmen and could have backed either of them up with another unit if needed. If I'd upgraded my Stags to Centaurs they still would have been awkwardly placed for trying to turn either of his flanks. Instead I chose to break through near the middle of his line.
Except through use of maneuver you got your Stags on not-his-spear. And I don't see how having Centaurs is a point either for or against either of our positions. Sure, Centaurs and Stags vs spears is a bad day for the cavalry. So you maneuvered them to get a better matchup, going for a mid-line breakthrough instead of a flank. My point was that I'm not sure that the points spent on ravenwood wouldn't be better spent, for example, upgrading Stags to Centaurs. I'm confused by your statement that in the above situation Stags was a good buy but making them Centaurs probably wouldn't help because there were spears on the flank. Stags solved that problem. Centaurs probably would've done the same maneuvering and been better at getting a breakthrough.
I wouldn't argue that a no-archer army can always be upgraded by adding an archer (and stealing points from elsewhere) but I think it's often the case that the archer version is at least as good and sometimes it's better.
I do see the point that you're making, that the advantage of archers is the ability to concentrate damage. Which is the main appeal of a Combined Arms army: it can get a breakthrough anywhere. However, almost by definition its going to do so in a more grindy manner than if you dumped the points into spamming or buying more expensive shock units (the goal in both cases is to win fast by pinching or just having a better unit). And if a combined army is more of a grindy army, the shooting is almost certainly going to be done mostly when enemy units are engaged, and the (-1)-0/-0 or (-0)-1/-0 engagement penalty hits that army build too hard (IMO). Because, IMO, it creates too many dead points to be a very good build and in most cases (not all mind you, not being absolutist in my statement here), it's better to spend your points on more units or better units on the line.
Most of this I'm not just drawing from my experience, but from games and session reports that I've seen. Perhaps I'm mistaken, but I don't see a lot of people used combined arms builds. That being said, I do agree with you and Niko that (wood/high) Elf archers are hurt less by the engaged penalty and those armies usually can do better at the combined arms because of it. Still not enough for me to do it often, but I guess we'll just disagree on that.
But aside from that, addressing the main issue of spam vs. CA, I must say that spam can work once or twice, but once your opponent knows that's what you're bringing, they become markedly less effective, suddenly the one trick pony isn't so attractive.
I don't think spam is a one-trick pony at all. Spam is just taking a lot of one type of unit. I've seen very effective builds taking stand and shoot (2 Bowmen + longbowmen), Close & Hose (I start with 3 Libyan Foot in most Carthage armies and adjust from there, and I once faced a very scary 7-wide principes+Italian Foot line), or even more insane builds (I think I saw Kevin take a line of Atlatlmen once; crazy doesn't mean ineffective). Heck, with Kingdoms once I had 12 Crazed Gobbos, 3 Ogres, and 1-2 Wolf Riders. There was no trick to that pony, just a hoof-full of crazy in your face.
