Microsoft Knowledge Base Article 816307 contains the following summary:
This article describes the best practices for using dynamic disks on Windows Server 2003-based computers. If you use dynamic disks, you can create fault-tolerant volumes (mirrored volumes and RAID-5 sets) and very large multiple-disk (or logical unit number \[LUN\] ) volumes by using striped and spanned volumes. These features are available only on dynamic disks. Dynamic disks are more robust and fault-tolerant in the way they store and replicate disk and volume configuration information. Dynamic disks are primarily designed to always be online. For this reason, they are not available on removable media. Follow the recommendations in this article to keep your data online and accessible.