You've heard the phrase - don't put all your eggs in one basket. Obviously, its good sense to diversify and protect what is yours. The concept crosses over from food into finances, and the fundamentals of public relations. This might seem like a strange position from someone who owns her own public relations company, but I struggled with working with only one large agency when I worked in the corporate world and have watched others grapple with it, too.
The concept of one-stop shopping is appealing, but if all your efforts - public relations, advertising, marketing, etc. are going through the same company - be prepared for some challenges. Some of the pain points I experienced included:
-larger companies can't always turn projects around on short notice
-if you become unsatisfied with the service you are receiving, severing ties can be difficult. Your agency is likely to be the liaison between many important subcontractors - such as graphic designers, web designers, etc.
-it is difficult, even for large companies, to be the expert in everything
So how do you work around this? Consider bringing someone in on a project basis. Notice I said "someone" and not "company." Adding on a competing agency will only increase your challenges with your current agency, and won't solve any of the problems outlined above. In my case, we discussed our decision to bring on an independent PR practitioner with our agency before we actually did it. We explained this was simply a new project we were given and we wanted to take the opportunity to have one person solely dedicated to this initiative. We weren't interested in severing our ties, we just wanted more options.