A reasonable objection to this is to question why someone would not enable Javascript, or download the plug-in for the latest media type. Every plug-in that's downloaded has the potential to decrease system stability. In a world dominated by Microsoft Windows, what little stability users can conserve is a precious thing, and should not be given up without careful consideration. The other problem with every new plug-in is that each is a potential security risk. The security flaws of browsers' implentations of Javascript and Java are well known.
Cookies have their own set of privacy problems.
As for plug-ins, millions of people downloaded the several generations of RealAudio's streaming media plugin before someone discovered that they sent tracing information back to Real Networks without user's knowledge. The same was true for Comet Cursors. It's a purely voluntary, nonevangelic movement. A simple Java applet can invisibly sap a visitor's system resources. It's as simple as that.