While both graphic design agencies and freelancers have their pros and cons, hiring an agency comes with some very distinct advantages.
The combined experience of an agency is a big advantage for a client as graphic design agencies will have teams of staff looking after a job, that means that there will not be any danger of sick staff posting a deadline. Agencies will also have more experience than freelancers in dealing with large clients and accounts and will usually have a long list of satisfied clients. Another big advantage of an agency is that due to their size, the agency teams can come up with a broader range of ideas than a single freelancer can. A design agency will have access to better resources then a freelancer with more extensive industry contacts and better equipment.
The real advantage of hiring an agency is that you generally know what you're going to get, while freelancers are far more unpredictable. I do not speak for all freelancers but in my experience the majorities are after big chucks of money and often do not think about the client's needs to the same level as an agency.
Graphic design agencies hire accounts people to deal with clients and manage expectations while a freelancer is more than likely to be a designer at heart and are generally not so good at building client relations. This can lead to missed deadlines and unreliable work ethic. I do not speak for all freelancers of course but there can be no doubt that agencies are better organized.
The dictate: Hire graphic design agencies over freelancers.