I know too many people who talk like they write
All the way back to the days of "Plain Talk" car insurance policies (a concept pioneered at Sentry Insurance by my favorite ex-husband, Mike Dry), this thing of good voice in writing, particularly business or ad writing, has been proven many times.
Kathy Sierra gives us the following article to back the idea and offer some nice practical tips. Creating Passionate Users: Conversational writing kicks formal writing's ass
Meanwhile, visit the Mother Site (author/source) of this blog: Faith River Communications for more on copywriting.