What effect does requiring a plugin have on conversion rate in a web app?

An HCI professor of mine found in a small study that requiring an added plugin (Java Silverlight ext) to view certain content resulted in a greater than 50% drop in users completing the task. Users saw the requirement, didn't want to install the plugin or otherwise did not complete the task once prompted.

Obviously this isn't the case for Adobe Flash or other extremely common plugins, but I was wondering if any research or statistics were available for the effect such a requirement has on users completing a task. It's obviously an added step which always leads to losing some users but requiring a plugin seems to be a particularly lethal turn off in web apps.