If you have a literal MTL tag, people will use it to blacklist chapters. It doesn't need to be supported officially by us, it'll inevitably happen. Then people will stop using the tag, because they're disadvantaged by doing so, and now you've driven MTL usage underground so people won't admit to using it. It accomplishes nothing except making it harder to tell when MTL is being used.
It's unfortunately a vocal minority. Bad quality translations, whether MTL or not, are consistently some of the highest viewed chapters on the site. The average reader doesn't care about the quality of the chapter they're reading, only that they can read it. As much as I personally wish people cared more, they don't.