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?

• Experts rely on experience to guide them, rather than explicit rules.  We have guidelines for how to write spam rules.  However, people like me will often refer to previous experience when dealing with a new spam run.  Just recently we ran into blogspot spam.  We can recall from previous experience that this is a lot like the Geocities or Tripod problem a few years back.
  
  
• Experts perform tasks automatically, rather than through (self-) conscious effort.  When we first started fighting spam, we had to consciously think about what we were doing.  The same goes for adding new IP blocks or even coming up with a new antispam strategy.  However, over time, the ability to evaluate pretty much anything spam-related comes automatically.
  
  
• Experts display significant efficiencies in processing information.  This is one of the distinguishing factors that experts in spam fighting have - we possess the ability to determine what the end results of our actions are.  We know what certain patterns of text are more likely to appear in spam than others.  We know how Coca-Cola's web page might not be a good thing to block.  We process multiple streams of information because so much of it is internally accessed.
  
  
• Experts possess multiple strategies for dealing with challenging situations. We don't just write spam rules; spam experts are expected to develop multiple skills for fighting the same type of spam.
  
  
• Experts approach problems more flexibly than beginners, relying more heavily upon intuition.  I have touched on this in the past on my blog.  Often times, I have trouble verbalizing why I think something is or is not a good idea.  It's not a gut feeling because it's honed from years of fighting spam; I can simply tell what is and is not a good idea.  I also know what can make an infeasible idea turn into a plausible one.

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.