<< With the Size/Resolution dialogue and Resample Image/Don't Resample Image what's the purpose in having the ability to crop or vice versa>>
The image resize dialog boxes will allow you to change resolution without affecting the actual image size in ins/mm/cm etc. So, if you wanted an image with a resolution of 240 to change to 300 for printing, the density of pixels will change but not it's physically printed size. So, a 12x18 image will stay at 12x18 no matter what value you put in the resolution dialog box. Remember to check "Bicubic smoother for enlarging or Bicubic Sharpening for reducing size.
You can use image resize to physically resize an image, but this may undo the proportion of the image if you do not check the Constrain Proportion dialog box. Than it's more than likely to be out of proportion and not 12x18. I always use crop to select my printing area set to 300 resolution.