This is a really bad suggestion.
Alpha channels take up as much space as RG and B combined.
(RGB get compressed together, and Alpha gets compressed solo)
so combining the roughness, AO, and metal together into RGB costs as much as adding one additional texture to the alpha channel of the Albedo.
Also, due to how normal maps are generally saved (BC5) you cannot add an additional texture to the alpha channel as the Red channel gets saved in the RGB slots combined, and the Green channel gets saved in the Alpha channel.
Unless you save as DXT which results in normal maps having much more compression artifacts, and not to mention you are wasting more space due to adding an additional alpha channel to your normal map.