If you always use perfect grammar when speaking, or even writing, you can come across as stilted. For example "Who are you gonna go to the dance with?" sounds much more normal than "With whom will you go to the dance?"
That said, there are way too many people whose use of grammar is so bad that it's difficult to tell exactly what they're trying to say.
"That said, there are way too many people whose use of grammar is so bad that it's difficult to tell exactly what they're trying to say."
I hope this doesn't sound too racist but I have to deal with some blacks every day who seem unable to pronounce words in a way that can be comprehended. Couple this with ebonics grammar and I often have to pause while my mind processes all this back into actual english and sometimes I am unable to do so. In the worst cases I simply have to smile and walk away wondering what it is they are trying to say. Amazingly they seem able to understand each other with little effort. Some of them obviously have two modes of speech and they are easily understood when speaking directly to a white person but totally incomprehensible when speaking to another black person. I refuse to use the term " African-American".