Having a mode in an editor that doesn't allow people to edit stuff is simply pointless.
The same may apply for a mode that shows how the result is going to look... Scribus is intended to edit the document, hiding the border of a selected item does not help editing, it only helps the preview.
I'm not advocating making the preview mode read only, I'm just picking on the arguments here. Your arguments can be applied both ways.
I have noting against the possibility to edit in the preview mode, what I am against is that the preview mode is requested to only show the frame of the selected object when it is hovered over. Then it is no longer a preview mode, but some mix of preview and edit mode.
To me, it makes most sense to have two modes:
1. Outline/working mode, where guides, margins, object boundaries etc are shown
2. Preview mode, showing the documents as it would look when exported.
Editing can be possible in both modes, but in preview mode if might be hard to select the correct object due to lack of visual feedback. But if the modes can be easily switched between you can select the object in working mode and then change it in preview mode.