The Pros of outsourcing Marketing is that the marketing company should be familiar with all the marketing techniques available. The marketing company should also be able to provide all the necessary statistical analysis to properly evaluate the marketing campaigns you launch.
The biggest Con to any outsourcing project is that nobody cares about your business as much as you do. Marketing is a very industry specific and company specific task. Asking an outsourcing company to know as much about your products, customer demographics, competition,and company goals make it next to impossible for the outsourcing company to do an adequate, personalized to your company, job. And that personalized experience is what marketing is all about.