[nknighthb]

Any time that mistake is made in something I'm reading, I have to read backward and forward several times to ensure I know which it was supposed to be. This requires additional time and effort.

At the very least, not making an effort to use the correct word is extremely disrespectful of your audience and their time.

[sokoloff]

This. Time and effort that you make someone else spend on your words is time and effort they no longer have to spend on your message/meaning.

When you're in a situation when your message is competing with others, the clearer wording will out-compete muddled wording. That could be in advertising, recruiting, pitching, evangelizing, or any number of places where you want to influence the reader in some way. Every Joule they waste puzzling out your words is one they won't use being influenced (positively) by your message.

How often do you see you're/your confused in professional advertising or press releases? Very rarely.