Why do you need a marketing agency to develop your web presence when your programmers and UI designer already know html?
If your company is pre-launch, having your development team whip together a basic site is probably fine. But modern marketing programs are very web intensive and have specific functionality requirements. Diverting your development team for building web pages for the marketing team not only slows your product development, but also cripples your marketing team’s agility.
Supporting multiple inbound marketing campaigns requires the ability to quickly create and easily test new pages. That’s not going to happen if your marketing team has to wait for a busy developer to free up time for every update. (Especially if the developers insist on coding each page individually or using a CMS platform that is a complete pain in the ass for non-technical users and has a less robust community than WordPress. (Drupal or Zope, anyone?)
Plus, developing for product functionality is not at all the same as developing to maximize marketing conversions. Your marketing teams will need to be able to quickly update pages to test messaging, offers and more.
If you want to maximize your website and campaign conversions after launch, getting help from an experienced marketing pro who can structure your site properly is a wise idea. Remember, crippling your marketing team’s ability to rapidly implement and quickly optimize marketing programs will cost you a LOT more in the long run than hiring a good web marketing design firm.