No plug-ins are necessary, file size is small so no modem is too dinky to spoil the effect, they can be viewed on any system, users can start, quit, rewind, and fast forward Viewlets anytime they please.
Online customers are only a click away from the competition.
When people come to your site to learn about you and your products, don't make their lives miserable by pressuring them to download a plug-in, or by making them wait, forcing them to register, or to read endless lines of text. They'll be a lot happier if you can just show them what they want to know with a few Viewlets.