Magic Lantern is not designed to replace Photoshop 
You are Right! Some of the videos with the current (height) selection are close enough that will require small crops after the stretching.
But there are some of them that could be interesting such us:
1320x740 requires a height of 442. The closest we have is 360 or 480.
If we chose 360 it does not work because when properly stretched it only gives 600. So the video aspect ratio is not 16:9
If we choose 480 it stretches to 480x1.674 = 803 and the video is 1320x803 which can be cropped to 1320x740. BUT!!!!
We are struggling to get the writing speed fast enough to do this, of course depending on card speed.
So we have 1320x740 with 480 as height requires 63.3 MB/sec
While 1320x740 with 442 would require 52.8 MB/s.
If you are developing Hacks to boost the speed, then it would make sense in some of these places to add the need height to lower the required writing speed.
The special cases that I have identified that could benefit from this are:
1280x720 the writing speed would decrease from 61.4 to 55 which is an 11.6% decrease
1320x740 the writing speed would decrease from 63.3 to 58.5which is an 8.2% decrease
1720x960 the writing speed would decrease from 115.6 to 99.4 which is an 16.2% decrease
1880x1060 the writing speed would decrease from 126.3 to 118.7 which is an 6.3% decrease
1920x1080 the writing speed would decrease from 129.8 to 124.6 which is an 4.2% decrease
Given that many struggle for writing speed then this might be worth the trouble.
Thanks for listening