the recipient is aware that the sender gave him more work to do than was necessary -- something usually not considered a compliment.
Yes, the recipient will be aware that you don't value their time enough to spend some of your own crafting a message that's straightforward to parse. It's worse than that though; particularly in electronic communications where you're often fighting for a limited slot of someone's attention, you've just made someone spend a bunch of time deciphering your meaning, reducing the amount of time they've got for actually considering your meaning.
Making a message easy to parse doesn't necessarily mean formal grammar - there are all kinds of short-cuts you can take and still be unambiguous, particularly when you know your audience well. In most business settings you probably want to avoid that kind of short-cut though, so it's worthwhile learning formal grammar if you ever want a job that requires a written application.
Does it matter if you use bad grammar in some settings? No. Does it matter if you are incapable of using good grammar? Yes, I think it does.