Probably never, unless you're also downsampling because you only plan to watch videos on your phone / downgraded your TV forever / are losing your eyesight. Los

Probably never, unless you're also downsampling because you only plan to watch videos on your phone / downgraded your TV forever / are losing your eyesight. Lossy transcoding barely makes sense when the source is a hilariously bad codec like MPEG2 (by modern standards), and even then you lose a lot of texture (the main benefit is inverse telecine and dealing with any other weirdness of the source disc ahead of time). You pay in quality for every time it's been lossily encoded, but only reap the space savings of the last (to a first approximation). You might save a little HDD space, but you'd have very poor quality for the bitrate.