Incorrect spelling, punctuation or grammar undermine the purpose of the writer, which is to inform, enlighten, or express an opinion. The writer appears to be unfamiliar with the written word, is poorly educated, or lazy. This suggests that he is not well informed, and hence, the reader is unlikely to value highly, the opinion or information offered.
This is not to say that one should be prejudiced, but that one is. The message is not rendered invalid by bad grammar and spelling, but it is devalued in the mind of those reading it. They have no way of discriminating between the writer who has deliberately chosen to write 'nite' instead of 'night', and the writer who knows no better.
They can't judge either, whether it has been done " due to a lack of ignorance ", or because of a high degree of ignorance...