Default nameplates use "HARMFUL|INCLUDE_NAME_PLATE_ONLY" filter for UnitAura to show only critical debuffs.
Like knowing about aoe stuns from other players is about the extent of what i need from nameplate auras, so simple option to mimic default behavior would be nice.

Correction:
Actually they use

local name, rank, icon, count, debuffType, duration, expiration, caster, _, nameplateShowPersonal, spellid, _, _, _, nameplateShowAll = UnitAura(...)

and

local function ShouldShowBuff(name, caster, nameplateShowPersonal, nameplateShowAll, duration)
if (not name) then
    return false;
end
return nameplateShowAll or
       (nameplateShowPersonal and (caster == "player" or caster == "pet" or caster == "vehicle"));
end

Sorry for taking so long, I didn't realise how important this was (i.e. what the default UI showed) until someone started ranting about it in IRC.