It really depends on where I see the bad grammar.
If it's on the internet, I usually don't mind if the person is still learning English. Obviously it's hard learning a whole new language.
Now, if they aren't still learning English, it's different. If it's on a forum or social media website that is strict on age, I can assume that the person is at least 13 and has been to school, and that they should be able to use proper grammar.
An old teacher of mine actually told a story about someone she knew who didn't use proper grammar because he was so used to using it all the time.
This guy she knows couldn't get into college because of his habit of using chat speak. He seriously used 'they're, there, and their' in incorrect ways, didn't capitalize any letters, and my personal favorite, he used 'lol' in part of it. Obviously this college denied him. So yeah, kids. Grammar really is important. If it stops you from getting into college one day, then it seriously needs to be worked on.
Bad grammar is really just a bad habit for those people who know proper grammar. But if the person is still learning, it's fine. It's just like learning any other language.