I just read a fantastic article by John Miano called, "The Myth of the Interchangeable Programmer: Can't We Just Offshore Him?".
Basically, the premise is that there's a very common misconception that all programmers are alike and have the same productivity. It's definitely worth a read.
While his article focuses on the practice of offshoring programming to cheap overseas countries, the idea is also relevant when talking about hiring new (local) programmers either to replace others or in response to increased workload.
The misconception that all programmers are the same can lead to an almost apathetic approach to hiring by managers. If someone quits, that's fine, we'll just hire another programmer to take over the work. If there are new requirements that mean we need a new developer, then fine, we'll just hire another one. One problem is that there's usually a significant lead time filled with learning about the project before even the best programmer can be truly productive. The other problem is that which is outlined in Miano's article; one programmer can be many times more effective than another.
I'm not sure what the solution to this is. I've preached this line many a time but because it's in such stark contrast to the person x time = fixed productivity formula that is relatively true for most other professions, it falls on deaf ears.