[ErroneousBosh]

You're not throwing away frames, though. You're blending the lines from the odd and even frames so that you get the full vertical resolution, with a certain amount of motion blur on moving vertical edges.

Analogue video is 25 frames per second, 50 fields per second. You could guess at what the "missing" lines in a field are, but that doesn't magically make it 50p video.

[LocalH]

The effective rate for 50i or 60i video, in terms of motion fluidity, is 50 or 60fps. Just because it's half the resolution per field doesn't mean that it's not a "frame" of video in abstract terms, just that it's not a "frame" in the jargon (because fields by definition are half a frame).

It is impossible to decimate a video from 50/60i to 25/30p without losing half of the motion fluidity, even if the properly interlaced source video is technically 25/30fps.