Well, in the general case, sure; but
Exactly. 
What we should be doing here is simply doubling up each frame; for some reason it isn’t doing that.
There are also some cases that using the whitelist can’t fix. For example, you remember all the work you did on playback of 1080i/50 VC-1 videos: you ended up having to use the “Deinterlace” setting by default, because using the “Auto” setting sometimes produces stuttery output when the video is frame-interlaced. I suspect that’s the same problem (effectively 25fps video, 50Hz output).
I’m all in favour of sorting out the whitelist logic (as previously discussed in the latter part of this thread) but I think the specific issue of why it can’t seem to convert 25fps to 50Hz reliably would also be worth looking into at some point.