I’m writing far more custom code lately (plugins and theme functions, really all the same) because when you start using wordpress as a full-fledged CMS, you’re dealing with bleeding-edge functions that have only barely been discovered by many (or are still waiting to be stumbled upon in the source itself). So there aren’t many plugins out there that handle these newest functions.
Also, the very nature of using wordpress as a CMS implies a certain specialization and customization for your client; thus even when I’ve found plugins that handle certain CMS functions, I almost always find I need a more specific solution. Plugins have to be written to be flexibile for all sorts of utilizations, whereas your custom code (which can be based on an existing plugin) can forgo all sorts of unnecessary input forms and settings panels which plugins require to know what you, individually, want as a result. Eliminating those (and hard-coding certain functionality for your client’s specific scenario) also reduces the amount of input UI and settings that someone may accidentally touch later…