SharePoint Designer is a great tool for developing workflows, because it brings the workflow development to the business owner, power user or information manager. You can quickly create a simple workflow without having to go through a often painful development process.
There are some limitations, of course. On a recent project, I really missed an action that would set the security of an item in a list, so that e.g. users could only see the documents they submitted.
Google to the rescue: this blog post explains how a developer could create a custom activity and make it available to the SharePoint designer users. Best of both worlds, isn’t it? Ease of use and extensibility?
Reference: http://www.codeproject.com/useritems/CustomActivity.asp