Oh, it's not really there as a specific counter, per se. Just something that seems like it would be obvious given the change in the effect. I could actually see it making some fights harder, even -- if you actually did something where berserk could cause unit density to flux, and by extension cause the units to spread out, I'd imagine it'd make stuff like 'zerker provinces notably more difficult and/or inconsistent. Do neat things to AoE/arrows/etc., too.
'Course, if you were actually playing with the mechanic instead of just having it as a static effect sort of thing, spells that specifically target certain densities -- not necessarily low or high, mind, but varying depending on the spell -- would be another obvious step, there. That cornered rat spell could grant bonuses the more stacked up units are, heat/cold auras (or fear) could have an adjustment based on density (probably not generally enough to outperform larger units with higher bases, but enough it's notable, and possibly synergies for specific units, too), all sorts of stuff like that. Certain priests or song-using units having a choral effect when close enough to each other (with density adjustments letting you stack more into an area) or somethin' would be thematic enough I'm suddenly wondering why something like that isn't already in the game (auto-commune when close enough to someone or X number of someones with the same trait?).
Almost certainly not needed from a balance perspective, of course, but imo unit density would be a hella' neat mechanic to be able to explicitly play with rather than it just being there or something incidental to an effect's impact.