I have to agree to disagree with you, but that is alright. I see no difference in placing a jpg tif or png and a pdf file onto a page to set up a bleed, I and every other designer I know do it all of the time with customer supplied artwork in InDesign the recognized leader of DTP apps. I do not want this to turn into a disagreement of whether or not Scribus should be InDesign, but I do have to argue that those of us in the business should be at the top of who are listened to in how DTP apps should/could be used. I have been doing this for twenty years now and I use a DTP app in this way at least a half a dozen times or more per week. Let me give an example to be clearer, I receive a file such as those file types I mentioned previously that is 8.75" x 11.25", standard US Letter size with 1/8" bleed on all sides. The said file does not have a technical bleed box for my prepress RIP to interpret, so I place the image into a Letter Size page with 1/8" bleed box setup and export as a pdf file that now has a bleed box that registers with my Digital Press RIP. Trust me, this saves a ton of time on prepress, production and post production items such as registration, cutting, and positioning. In the printing business, time is most definitely money and this option I just explained does just that without having to go back to your customer, who does not understand any of this technical necessities, and allows me to get the job done. So in my opinion, it is a prepress DTP issue, but that is all it is, my opinion.
Cheers,
mnawij