I can't help myself; I often correct the spelling and grammar of others. A common retort is: "Why does it matter if I misspelt that word in a comment/text message/email? You knew what I meant!" The reason it matters is that you are training yourself to do the wrong thing. The sloppier you are in casual communication, the greater the odds that you'll mess up when it actually matters.
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I recognise different modes of communication demand different voices. The way I write a story for Lifehacker is quite distinct from how I express myself on Twitter or in a text message to friends. The vocabulary, tone and structure will all vary. But one key element won't.
In all those contexts, I use capital letters and punctuation correctly, and aim for accurate spelling. I don't say to myself "this doesn't matter". It always matters. To gain the habit of accuracy, you need to aim for accuracy all the time.
Nobody is perfect, and I make mistakes. Everyone does. I write for a living, so I probably make more mistakes than the average person. However, I don't pretend that those mistakes are irrelevant. The people who make errors and then laugh them off, or argue that it's much more appropriate to use nothing but lower-case letters and the wrong version of its/it's when sending an SMS, are making a much bigger mistake. They are establishing the habit of deliberately being indifferent to accuracy.
When they need to write in a more formal context -- a job application, a vital work email, a message of condolence -- they will struggle badly. If you've long since stopped paying attention to how to use apostrophes in more casual contexts, it's far more likely that you will make a hash of deploying them. If you don't pay any attention to suggested spelling corrections, or use an environment which doesn't offer them, errors will be more common.
In a utopian world, human beings would be able to switch seamlessly from one communicative context to the other and languages would be constructed in an entirely rational fashion. We do not live in such a world. Writing accurately requires effort, both in working out what you want to say and in remembering the weird and often arbitrary rules that apply when it comes to spelling. In that context, practice definitely makes perfect.
You may not be able to change the habits of your spelling-indifferent Facebook friends, but you have control of your own keyboard. The choice is yours. Choose wisely. Accuracy matters.
Lifehacker's Mind Your Language column offers bossy advice on improving your writing.