[Duugu]

Originally Posted by Duugu The reason is, if you hide them all there's nothing left for these units. The standard names/guild names/whatever are disabled if nameplates are enabled, or not?

I did a short test to prove this. It's exactly as expected.
Enable nameplates, place youself in front of 2+ units. Target a unit and use the following code snippet

for i = 2, WorldFrame:GetNumChildren() do 
    local frame = select(i, WorldFrame:GetChildren()) 
    local glowRegion = frame:GetRegions()
    if glowRegion then
        if frame:GetAlpha() < 1 then 
            frame:Hide()
        else
            frame:Show()
        end
    
    end
end

Nothing left except for your target. Not very usefull imho.