I agree with what you're saying about online sources being unreliable, depending on the region (PAL vs NTSC) censorship in certain releases, extended editions etc. and it should absolutely not write into that field if it's a video file where MC18 can read the duration directly.
But having at least some approximation of time is a lot better than nothing - there is no duration for .iso files at all.
Duration for me really only has to be approximate when it comes to films - is this film less than 90 minutes, is it around 2 hours, or is it a really long film that is 3+ hours? (like Kingdom of Heaven where my Blu-ray is 3:09:33)
Sometimes I might not have time for a film that's over 2 hours long, and I want to watch something that's less than 90 minutes, for example.
Or I might have a spare couple of hours, and want to pick a title that is as close to that as possible.
And actually, just loading up Kingdom of Heaven to find that out, has let me see that the [Duration] field is actually not editable at all for iso files, so you can't even enter it manually if you wanted to right now. (I thought I might as well, considering I bothered to check it)