In my opinion, individually sized hidden frames are useless. Why do I think that?
I work with hidden frames very often. When I use them, my main aim is to have some parts being alone on white background without anything around – people should just focus on this part and not be annoyed by any other “focusbombing” objects.
Have an example (img 1 of the attachment): I always give my objects some padding as it looks much neater. I therefore draw my hidden frame around my text (img 2). When I try the prezi, some other parts can be seen that in fact shouldn’t (img 3).
In such a case I have to go to path->add current view, delete the added pathnumber (because usually they’re wrong and I’m giving them another number anyway later on) and then move the hidden frame, which now has the correct ratio of my monitore and makes finding a “clean” position for my isolated parts easier, until I find the perfect position.
Why not draw hidden frames with correct ratio automatically?