Tbh I have done some testing on the effectiveness of increasing precision of my archers, but it's extremely difficult to really get a intuitive grasp on how much it matters from simply watching it play out because as you say archers are normally a large scale thing. On a small scale archers even with a lot of a precision buffs will still miss a lot. On a large scale it's impossible to keep track of the increases in efficiency without more effort then I'm personally willing to put in, so I guess really only like... A LOT of testing and looking into the ultimate results would really help, more then I was willing to do at the time.
Here's my take on it though:
Precision, except for very high amounts of it, doesn't matter too too much. It sets a range where you have perfect (not actually perfect because there's a DRN roll) accuracy, which is where archers and mages are very deadly, but this range starts out very short and you need a lot of +precision to make it large enough to get an extra volley/cast an extra spell as the enemies close most of the time. Theoretically if you have ways to slow the enemy or otherwise keep them in or near this area, +precision might matter more, and there's theoretically break points vs specific combat speeds where precision gives you an extra "perfect" accuracy volley. But I have no idea where those break points are.
Outside of this perfect accuracy zone precision will lower the amount of deviation your shot will throw, but that deviation is on a bell curve anyway, not a hard number. So the actual gain in precision is... Hard to tell? If you're trying to hit a small target it will help more, but you're also likely using the wrong tool, if you're trying to hit a fairly wide area of squares, it'll help less and I think is, as you sorta said, matters less, firing into 200 dudes isn't going to be helped enough by +2 precision to make it worth paying more for it for the most part.
My ultimate take away is pretty much what you started with. Use archers in massed battles (or, often, not, since a lot of times archers suck) Use mages on advance and cast spells