Now that I’ve played around a lot with what you did, I think I understand why you think it’s so simple. Your shader seems absolutely optimal, but only if you only use what is effectively a 1-bit alpha. I’m imagining a variety of greyscale values in the alpha channel, based on how much you want any given area to be colorized.
Up to now, I had been thinking about the alpha channel as a mask for transparency, for the RGB image, and applying that on top of a “colorized” version of the same image. (This is exactly what is going on in my shader.) However, you can see from my screenshots below (my previous method on top, yours below) how much more practical it would be, given a large amount of layers, to set things up using your idea. With my old method, you have to keep merging layers and putting the result at the bottom. That is stupid :o, but I hadn’t used it enough to realize that.
Now, it would be no big deal to invert the mask used for the color to be multiplied in, as an alpha channel. And I think that it should be, if using your shader, to avoid the subtraction. But this is a non-issue, when using lerp as I do below.
See, I was testing out just the first texture block of your code, with the rest commented out, and had no idea this clamping would occur. So indeed, blown highlights are not an issue, but there’s still plenty of weirdness going on if you use any grey values in the alpha channel.
See the second line in my first post in this thread. It’s currently the last post in that thread. But I think I improved it, using your logic:
Now that I’ve embraced your view on the alpha channel, it’s a simple matter of switching the order of the first and last arguments of the final line of the shader. So we don’t even need to do a subtraction! 
Shader "Tinted using -Alpha" {
Properties
{
_Color ("Tint Color", Color) = (1,1,1)
_MainTex ("Texture", 2D) = "white"
}
SubShader
{
Pass
{
SetTexture [_MainTex]
{
ConstantColor [_Color]
Combine texture * constant
}
SetTexture [_MainTex]
{
Combine previous lerp(texture) texture
}
}
}
}

