As a developer I wrestle with this all the time.  Should my clients go with custom or off the shelf?  As the computer revolution matures, it seems the answer is increasingly off the shelf for more and more companies.  I don't like that this is where things are going because a) it means more and more control over our lives by fewer and fewer people and b) less work for me.  But the economics of the situation are hard to resist. 
Also, I find that, sometimes, even when the client thinks their business processes are different, they really aren't.  I have worked on custom applications and only later discovered that the whole rationale for the project was based on a faulty understanding of the client's business model -- by the client!  Once their real needs have been revealed through a long process of back and forth, it becomes apparent that an off the shelf solution would have ultimately been a better fit.