Poor grammar is ugly. To any lover of the English language who values a correctly constructed sentence, it can signify a lack of intelligence or effort. Are many of us limiting ourselves in terms of potential success by u'sing apo'strophes every time we see an 's?
Much of the problem stems from the fact that there are two schools of thought on the matter. You have those who are for teaching prescriptive grammar and nothing but. They insist on a strict set of rules with no wiggle room. Ending a sentence with a preposition (see "but" above) is tantamount to murder. Split infinitives are the devil's playthings. But then along came the new generation -- now descriptive grammar is the way to go. Allow for some flexibility and more accurately reflect the way we express ourselves. It shouldn't be a federal offense to start a sentence with "and" or "but." Go easy on the split infinitives -- they never hurt anyone.
Because of the confusion and controversy (perhaps this is a matter that should be slugged out on an episode of Hardball) the line between acceptable and unacceptable grammar has become a bit blurred. Even though the rules may have changed, they do still exist, and there are times that following them is especially vital. Here's a little guide to help you remember when it's important to exercise a bit of responsible grammaring (← not a real word):
Instances in which good grammar is essential:
1. You are filling out your college application. If the folks at Harvard catch you making an unnecessary shift in tense, they may just shift your application to the bottom of the pile.
2. You are writing the essay portion of the SAT or ACT. There's a reason this section doesn't have bubbles for you to fill in. They want to know if you can write. They are already quite satisfied with your "coloring-in" skills.
3. You are sending an SMS to your girlfriend asking her to marry you. She is going to be peeved enough that this is how you chose to pop the question -- no sense in ticking her off more by displaying a lack of subject-verb agreement.
4. You're penning a speech for the President of the U.S. to deliver on-air. If the American public hears our leader use a dangling modifier, everyone's going to start doing it.
5. You are applying for a writing gig at Shmoop. At the minimum, try to spell our name correctly.
Times when good grammar may not matter quite so much:
1. You get a gig writing packaging content for Frosted Flakes cereal. You're grrrreat!
2. You are Mark Twain ("I didn't do him no more mean tricks, and I wouldn't done that one if I'd a knowed it would make him feel that way") James Joyce ("Must have been that morning in Raymond terrace she was at the window, watching the two dogs at it by the wall of the cease to do evil") or Anthony Burgess ("I had to have a smeck, though, thinking of what I'd viddied once in one of these like articles on Modern Youth, about how Modern Youth would be better off if A Lively Appreciation of the Arts could be like encouraged"). None of those people? Then you should probably cross your Ts and dot your Is like the rest of us mere mortals.
This is just a sampling of the grander smorgasbord of literary situations, of course. Basically, you should follow this general rule: construct your sentences carefully and with great attention to detail any time you don't want your intended audience to think you're plagiarizing straight out of Huck Finn.