I can really appreciate your point "When you're an adult, nobody corrects your grammar once it's good enough to be understood." It's true. I speak near-fluent Russian as a second language, but it's hard for me to get better because people are too polite to correct my mistakes. If my idea clear, people don't want to appear impolite.

I can't be too critical: I do the same thing when people speak broken English to me. If the idea they want to express is clear, it seems improper to correct them.