I understand your point and they still are not the same thing, I simply have to assume you've never read the source code for image search from ahk or have used image search yourself.
Pixelchecksum has its uses, want to know if the pixels in an area changed? Great, use it. That's what its for. Looking for a particular picture on screen, and in most of our contexts, in game? Well good luck with that. It takes a decent amount of tolerance when matching images and you want to use a function that returns a strict checksum to determine if your image is on screen. I've not played with the function but even trying to compare the same image to itself with image search it never matches with 0 tolerance. With image search you can define any color to be transparent and ignored in the matching and you're not going to get that with pixelchecksum.
Also, to use the checksum, you're going to what? Screen shot, and cut out the image you want and then get your checksum for the image? Then set a variable in your main script and look for it. Ok, so is this image you're looking for in one place all the time? No? Ok so now you're going to write loops to search the whole screen or areas of the screen? If your image that you got a checksum for can appear any where on screen or in a smaller area you're going to have to start searching in one spot then increment over 1 or 2 pixels at a time, reach the end of the line from left to right and then if needed move back to X origin and down the y axis to continue the process. I'd be willing to wager money that is far more time intensive than just using image search in this context.
I'd be more than happy to see a coded example that proves the built in checksum search would be faster to find an image on screen than image search.