first things first, I just need to specify: almost all of the translation projects with people that actually know the language basically never have mtl that isn't heavily edited to the point of basically being rewritten.
the main problem with using mtl without knowledge of the language is that you can't really ever know if you've got the right reading. just to list off a few examples: is there a proper noun messing with the output? are you encountering the hydraulic ram / water sheep problem? what about pronouns? references to other media? slang? or has the translator just eaten half the input since it couldn't parse it properly (surprisingly common for the mtls that value grammaticality the highest)? sure, you can solve some of those issues with a bit of googling, but can you solve all of them? what about the ones where the definitions aren't available anywhere in a language you know?
the reason that translators still use it sometimes is that a lot of those go away when you can manually correct the errors. not all of them, mind you: depending on the workflow and how bad the results are shaping mtl into a translation that reads well takes more work than just translating from scratch. however, in certain situations a quick and dirty lookup on a phrase dictionary with a search entry works as a first draft, and that's what you're seeing people talk about