A colleague, Ray Wang, has loads of fascinating statistics in his recent data overview, "The State Of Enterprise Software Adoption: 2007 To 2008." A staggering number of companies have developed and maintain custom applications.
Ray's piece is written for the market researcher, but like all good research, it has much broader implications. Perhaps you won't be surprised when I say, "Including product managers!"
The statistic says that a lot of demand exists that technology vendors are not meeting. Custom development is a last resort--and I should know, since I've done it myself.
Twice in my career, I've pushed to create custom requirements management (RM) applications. In the last case, I wrote it myself in PHP and MySQL. (Never fear, it's a laughably rudimentary application, functionality-wise, so the friend and colleague who inherited it wasn't left with an unmaintainable mess.)