Loose adherence to grammar rules doesn't bother me much in any writing, to a degree. I'd rather have a lively, interesting text than a stuffy one that's correct because the author didn't split infinitive or make sentences without a verb. At some point, too much grammar kills creativity.
However, I intensely dislike bad spelling and really childish grammatical mistakes: yes, the main message is conveyed across, but the text constantly delivers another message, which is "the author didn't bother to learn English", that simply detracts from the pleasure of reading.
Unless of course it's done on purpose. The narrator in
Feersum Endjinn from Iain M. Banks, for instance, is dyslexic, and the text is written as if written by somebody with dyslexia. The first pages are truly atrocious to read, but surprisingly, after a while, it gets easier and easier until the reading almost flows naturally again. Interestingly, the book works, which is another proof that Iain Banks is a genius
