This is a great idea, that I've often needed. Sometimes, when I have linked multiple PDF's of different which have some overlap, not being able to crop makes it a pain to adjust which PDF is above the other.
I would add, it will be good to modify PDFs and images appearance. Apart from cropping, changing brightness, removing background and other tools need to become available as it will speed up the creation of some drawings.
I personally, wouldn't add PDFs to revit files at all, but sometimes recreating info from a PDF is rather pointless.