This is a point I have touched on a number of times in the past. One of the selling points that I point out about outsourcing your spam management to someone else as opposed to doing it in-house is that when it comes to spam, you really do want an expert managing it. Spam is constantly changing; there are piles upon piles of corner cases to consider and the person dealing with it should be an expert. We have several of them here in this company.
What makes an expert? One of my other interests is learning to trade stocks. I found a blog post by a writer I regularly follow about what makes an expert trader. There are no set guidelines, but there are definitely behaviours that expert traders have that others do not. I believe that this applies equally well to fighting spam. You might have a guy on-site dealing with your spam, but he won't be an expert. He just doesn't see it every day.
What are some qualities of expertise?
I can appreciate a company's desire to not want to spend the money and do their spam fighting in-house. I wouldn't recommend it; spammers are actively trying to defeat you. They mean you malintent. You definitely want somebody on your side that knows their tricks and is hardened by years of experience.