i agree it's a weird restriction...i remember at some point matt mentioning a reason but i can't remember
i've gotten around it by using import folders that ONLY look at audio or video, then when they are moved to the main library folders (which import EVERYTHING) the .jpg, .pdf, etc go with them and then are imported into MC as normal for me to view. it actually works really well, just took a bit of setting up to get it all proper.