The only way you can learn something from your mistakes is when somebody corrects them. If you say an incorrect sentence, and if someone points out the mistake and tells you the proper way to express your meaning, then your knowledge of English increases.
You may therefore think that speaking or writing with mistakes is not so bad because it is a chance to fix your errors and learn something new. Unfortunately, this is true only if you have a checker — a competent person who will correct your mistakes. If you don’t have a checker, speaking or writing with mistakes will only give you bad habits.
If you’re thinking that an English teacher could be your checker, consider this:
Many teachers (especially non-native speakers) are simply incompetent. Sometimes they will find “errors” in correct sentences or suggest bad alternatives. I couldn’t count the number of times it happened to me in high school.
Most teachers correct only the biggest mistakes and ignore the rest. The reason is simple: class time is limited. If they corrected every mistake of every student, they wouldn’t be able to teach their lessons. You could get some kind of private tutor, but how much would it cost to have him correct everything you write and say in English?
What about “random native speakers”? This category includes any native speakers who are not your teachers: your American friends, Australian penpals, native speakers on Internet forums, or just regular people around you, if you are in an English-speaking country. Can they be effective checkers?
Not likely. In general, native speakers will not correct your mistakes! As long as they can understand what you mean, they will completely ignore all your mistakes in grammar, vocabulary and pronunciation. For example, the message below is perfectly understandable and could easily pass uncorrected: