Here's the thing
The reason not to worry overmuch about the way people talk or write in e-mails or other casual contexts is summed up aptly in a short passage from Steven Pinker’s The Language Instinct, which I have been reading in short takes at lunch for several weeks.
The aspect of language that is most worth changing is the clarity and style of written prose. Expository writing requires language to express far more complex trains of thought than it was biologically designed to do. Inconsistencies caused by limitations of short-term memory and planning, unnoticed in conversation, are not as tolerable when preserved on a page that is to be perused more leisurely. Also, unlike a conversational partner, a reader will rarely share enough background assumptions to interpolate all the missing premises that make language comprehensible. Overcoming one’s natural egocentrism and trying to anticipate the knowledge state of a generic reader at every stage of the exposition is one of the most important tasks in writing well. All this makes writing a difficult craft that must be mastered through practice, feedback, and—probably most important—exposure to good examples.
Editors, copy editors, mavens and scolds, focus on what’s most important.