You would use DynamicLink anytime you want to create something in After Effects for a project you’re working on in Premiere Pro and Encore. For example a fancy title in Premiere Pro, or an animated background for a DVD menu in Encore.
The key to understanding DL is that Premiere Pro and Encore aren’t doing the rendering. What they do is create a “tunnel” to After Effects. AE does the rendering and feeds the frames back to the host program. (If AE isn’t already running, the host launches version of AE that runs in the background.) This type of technology is sometimes called “frame serving”.
So you can see how it doesn’t matter that Premiere Pro can’t use your plug-ins. It doesn’t need to. AE is doing all the work, it just hands the rendered frames back to Premiere Pro. To Premiere Pro and Encore, the AE comp is just another movie file.