That's another problem of mine. I tend to almost completely rely on regular armies for fighting
Hate to break it to you, mate, but you might be a scrub.
but units aren't that important anyway, later on. It's the mages that matter, and when they're sitting around, they're researching.
Oh come now, as someone who's general strategy tends to rely on bog standard units, I can tell you this is flat out untrue.
I have (figuratively) trampled over other armies with hyper-buffed infantry before after all. It's always fun to watch all the battle magic do absolutely nothing to deter my ranks. It's why I like earth magic so much.
And even if you prefer using mages in the traditional death dealing sense, you still need a healthy amount of infantry for your armies to act as ablative armor for those mages.
But at the same time let's not forget Reanimation style chaff spam, there just hits a point where there's just too many of them to deal with regardless of what you have.
Note how all of those strategies rely on magic, though. You need mages to buff, in that context, units are important as vehicles for your buffs, not for what merit they might have independently of magic. The "traditional" magic, which as you describe it is basically evocation, focuses on the mages still and uses units as a screen to protect the mages, who are important. I actually took this one to a bit of an extreme in my latest AAR, but even for something like Abysia, this becomes true soon enough anyway. Well, that specific example has trouble as soon as someone researches Rain, but you know what I mean. Reanimation is also done by mages. Even freespawn nations don't use that later on. Lemuria tends to stick their ghosts in forts and rely on thugs and other shenanigans later on, and Ermor uses them a bit more but still relies primarily on lictors, which are a vehicle for a big bless – essentially equivalent to buffing, though it comes online earlier – and still want some big mages in the back line, but are probably the best argument I can think of for not relying heavily on mages in any situation after the earlygame.
That's the kind of stuff I was talking about, anyway. It's relevant in the previous discussion because although you need units to fulfill some purposes, you can generally get them close to as fast as you need to, or else the reason why not is because of small size that requires you to expand anyway. Mages are a different matter, since they're limited to one per lab (and, usually, fort) per turn. They benefit from infrastructure, and more importantly they benefit from research. And they are the limiting factor, because you can lose a dozen turns at once and be in a very bad situation, whereas units can be effectively replaced much more quickly, and even if you're suffering losses, it's unlikely that your demand for troops will outpace your supply of them in the same way that happens with mages.
I once tested my Quicksilver Stampede against a max indie strength level 3 throne province full of undead.
There were the 5 priests on my side, sure, but still. They made quite the show. They chew up flak undead like there's no tomorrow, even ethereal ones.
(the satyrs mostly survived because they were stuck behind the wall of centaurs; the ones that died found a way around)
Yeah, sacreds are great. Even so, they don't scale like mages. Anything that counters 100 will have a pretty good chance of countering 1000. Assuming it's in a single battle, raiding's another matter, and numbers help there. But in a stand-up fight, somebody's going to find out how to whup you, and it's almost always going to be "loads of mages". Sometimes it'll be an SC, but their heyday ended with Dom3.
Then there are a lot of fools in the Dom4 MP community. Which, IMNPHO, there are. So, yeah. There are subcommunities out there which obsess about "dueling" in full-size games and dismiss diplomacy as weakness and a crutch. And OMG, they scoff at the idea of "defensive" play; it's all-out or nothing. It's almost charming in its simplicity...
Well, I don't think there's anything wrong with doing things like "dueling" if that's how people play in your community and you do it for fun. It's not playing to win, but if that's how you like to play, it's a perfectly legitimate choice. It's definitely suboptimal though, at least if there isn't a gentleman's agreement to not interfere with these duels. In a free-for-all (that is, no such agreement) winning without diplomacy is a possibility only if you're a skilled veteran and your opponents are very much not.
I'd argue that having units sitting around is not always pointless, though. If they're idle but deterring, they're not idle. Take as a rather extreme example: round 4.18. I had literally hundreds of troops (and by the end, thousands) sitting idle for almost the entire game, by design. Ofc, these troops were ghouls, and they'd die to banishment spam just about as fast as the longdead or soulless I was also amassing while being more "expensive" to reanimate. And they were "sitting idle" in forts where they promised to make any attempts to siege me down slow and miserable, and that was even if the defenders didn't break out BVCs. Again, that's an extreme case. But if you can secure a border by having idle troops and reduce the odds of your one-front war turning into a two-front war... mission accomplished.
I sort of agree with the thrust of this, and vampires especially are a good way to make nobody want to touch you. Ghouls are a good garrison, not only for deterrence but just so that if a fort gets sieged they'll have something that's not mindless in there to keep from being taken down in a turn. Something like that isn't a gain for you though, it's only a status quo, a lack of loss. Which is good, but it can't be your sole strategy, and the longer you spend only doing that, the further behind you'll fall.