[CoreXtreme]

Yes, they can certainly help in cases where you are offering your images to others.

Assuming, initial size is zero. If you install 100MB of packages in layer 1.

Then in second layer, you uninstall of those packages. Your image size will still be 100MB.

If you are optimizing for high density, a smaller size certainly helps.

[amq]

Others are very likely to already have the Debian base image cached - by using pretty much any official library image.