[laukond]

local function ActuallyHideGrid(barName)
    for i=1, NUM_MULTIBAR_BUTTONS do
        local button = _G[barName.."Button"..i]
        button:SetAttribute("showgrid",0)
        for i,region in pairs({button:GetRegions()}) do
            if region.GetTexture and region:GetTexture() then
                region:SetAlpha(0)
            end
        end
    end
end
ActuallyHideGrid("MultiBarBottomLeft")
ActuallyHideGrid("MultiBarBottomRight")
ActuallyHideGrid("MultiBarRight")
ActuallyHideGrid("MultiBarLeft")

This did not work for me. It would bug out one button every time I tried, and if I tried moving the spell on that button it would show all grids.

hooksecurefunc(getmetatable(ActionButton1).__index, 'SetAttribute', function(self, attribute)
    if attribute ~= 'showgrid' then return end
    self:SetAttribute('showgrid',0)
    if not HasAction(self.action) then
        self:Hide()
    end
end)

This on the other hand worked very well, BUT the problem is it is impossible to put at spell from the spellbook etc. down to the Action Bar, since the textures never appear.
So if there could be added a condition to only hide when not dragging anything (such as a macro, ability, trinket, etc.) then it would work very well :-)