The way you present yourself or your business is a direct reflection to the business community at-large of who you are. A quick glance at your web page can alter someone's perception of your level of competence. I see it all the time in local papers, large websites, and even The Wall Street Journal...grammatical and spelling errors, and if I'm able to spot one, it instantly alters my opinion of the writer and, sometimes, the business as a whole.
One example I can give you is a recent interview of a former collegiate professor of mine. This is an accomplished man in his field who is the chair of his department, and he hails from Argentina. His surname is pronounced (gah-sho), but it is spelled "Gallo" which would lead most Spanish-speaking people to believe that it would be pronounced (gah-yo). Argentina, however, uses a dialect of Spanish similar to Castellano in Spain and their "ll" sounds like a "sh". The interviewer used neither pronunciation, and I'm assuming she didn't bother to ask him politely how he pronounced his name, but she opted for the Americanized (gal-low) pronunciation when that English short "a" vowel sound doesn't exist in that language. In no way did this seem to bother Dr. Gallo, but to the listening audience which included myself, it sounded careless and unprofessional.
I've perused many websites, and scholarly journals, and other media. There is often a rough draft quality about many things I read. I often think to myself, "Clearly, writing in a public forum is a component of this person's job. Why aren't they better at it?". The bottom line is this: The devil is in the details. If you want to affect public opinion with the written word, come with your "A" game, and read what you write aloud to yourself before posting it, or allow a colleague to read over your post. You'll be glad you did. You may only get one new customer out of a thousand, but sometimes it's a high-profile customer, and it could be worth it.