Ok, they don't suck all of the time. Plugins are an essential component of open source applications, but you should be able to deliver a robust website utilizing the core of the CMS, relying on plugins only for project specific needs. The old 80-20 rule (and better yet 90-10) should be in play, 80-90% of the project functionality should be available in the base CMS, with 10-20% coming from third party or custom plugins.
I came across a blog post recently in which the writer bragged about the 32 Wordpress plugins he had running on his site. He dedicated the post to detailing each plugin so that other Wordpress users could take advantage of his good fortune. Lucky for them!
An informal poll released in Jan 2010, with over 1300 votes, shows that 33% of these Wordpress sites are using 11 or more plugins, and that is probably because they are running robust websites, and not just a simple blog.
Other open source CMS's have plugin envy as well, including Drupal and Joomla. Here's a post touting the 25 most essential Drupal modules. So it's fantastic that these CMS's have a robust community that is contributing add-ons for the application, but this blessing can also be a curse. Your site is at the mercy of the quality of these plugins. Every time an update to the CMS is required, if the plugin developer is not keeping up with the releases, and this happens frequently, your site is in danger of not working or worse.
These plugins can also pose security issues, putting your site in the hands of developers you may know nothing about. In fact, here's a post with 18 useful plugins to protect your Wordpress site. Great more plugins!
We deploy our client projects as hosted solutions, and supporting our clients is critical to the success of our company. If we had to test 10, 20, 40 plugins every time we issued a new version of the CMS, our projects would be extremely difficult to support and I am sure our SLA's would suffer.
Instead, we have chosen an open source CMS that allows us to deploy large scale, complex websites with few, if any, third party plugins required. For example, we launched www.softwaremag.com last week. This site includes:
Plugins can be very helpful, providing important functionality for a project that you would otherwise have to develop on your own. But we also see a lot of downside from a scaleability, security and support perspective. At the end of the day, a strong development team should be able to support your site and all of the plugins you require, but if you don't have that team behind you be careful, because plugins can suck!