This comment is pretty much what I was going for. I've reworded it to make it clearer.
The issue you can run into in practice is stuff like softsubbed signs, which can clash and look out of place with the native video if you render them at full res. There's also a related issue, which is that if you're using something like motion interpolation (e.g. “smoothmotion”, “fluidmotion” etc. or even stuff like MVTools/SVP), softsubbed signs will not match the video during pans etc., making them stutter and look very out-of-place - the only way to fix that is to render them on top of the video before applying the relevant motion interpolation algorithms.
Personally I've always wished for a world in which subtitles are split into two files, one for dialogue and for signs, with an ability to distinguish between the two. (Heck, I think softsubbed signs should just be separate transparent video streams that are overlayed on top of the native picture, allowing you to essentially hardsub signs while still being capable of disabling them)
Also, sometimes, rendering at full resolution is prohibitively expensive, e.g. watching heavily softsubbed 720p content on a 4K screen.
The issue you can run into in practice is stuff like softsubbed signs, which can clash and look out of place with the native video if you render them at full res. There's also a related issue, which is that if you're using something like motion interpolation (e.g. “smoothmotion”, “fluidmotion” etc. or even stuff like MVTools/SVP), softsubbed signs will not match the video during pans etc., making them stutter and look very out-of-place - the only way to fix that is to render them on top of the video before applying the relevant motion interpolation algorithms.
Personally I've always wished for a world in which subtitles are split into two files, one for dialogue and for signs, with an ability to distinguish between the two. (Heck, I think softsubbed signs should just be separate transparent video streams that are overlayed on top of the native picture, allowing you to essentially hardsub signs while still being capable of disabling them)
Also, sometimes, rendering at full resolution is prohibitively expensive, e.g. watching heavily softsubbed 720p content on a 4K screen.