You could but it would waste space. In my setup, I only need a single pixel to serve as the multiplier, but if I added a whole new layer, that's width x height more pixels.

Do keep in mind that a floating point buffer is 4 times the size of a normal buffer.

And an alpha mask can be gray scale, or even one-bit color.

Not that this couldn't be a good use of floating point textures for other reasons (A full HDR pipeline for example) But it sounds that using floating point textures is actually the real waste of space.