No doubt, Wordpress is the most popular blog/cms application. But when we first started using it to build sites (it was from 1.2 version I think), WordPress was a very different application. It offered only basic features and was meant just for blogging. Our needs were different. We wanted a cms application so we started hacking it. We liked a lot the administration part and the fact you could easily add plugins to extend it’s basic features. But most importantly we could code our own themes and create a different experience on each site we built. As blogging became very popular, WP matured in a far more complex application thanks to its openness to the users community and wise management by its creator, Matt Mullenweg.
WordPress based its growth in the abundance of plugins and themes. It’s relatively easy to write a plugin or create a theme. That makes WP easily very extendable and customizable. But plugins and themes can behave, at times, unpredictably. They can consume your hosting server’s precious (and expensive) resources. Most of them are free which means that they come with basic or no support at all. And since they are open source they can be potentially be exploited. So yes, plugins can add important features but they can also have bugs that can slow down your site and, in some remote cases, they can introduce security issues. So it’s always useful to use caution before adding a new plugin. It is wise to read the support and reviews pages of each plugin on WordPress.org, before deciding which to install. Choose the most popular not the most feature rich and always try not to overuse them.