Do you want more clients? Are you seeking more speaking engagements? Do you want to see sales of your book take off? Write like a professional.
Whether you like it or not, many people pay more attention to how your words make them feel than to what you actually say. If poor grammar, spelling errors, improper punctuation, foul language, or inappropriate humor are present, you may be deemed unprofessional even if you are THE expert in your field.
When in doubt, ask for help. Get input from a trusted friend. Hire a copy editor. Write like a professional.