Indeed -- the notion is that one could do things like put support for encrypted/signed pages or other cryptographic tools (I haven't checked if Netscape mail handles plugins, too) directly into Netscape where ordinary users could smoothly use them.
Doing tech support at my ISP has made me very skeptical of anything that requires a plug in. Most people aren't willing or able to download and install them. Obviously that's not a problem for large organizations who want to run something internally -- they can make people install them and provide support to make sure it happens. But if you want to publish to the mainstream of people who use the net, using a plug in is a very bad idea. (I don't know anyone that doesn't design web pages for a living who's installed the shockwave plugin, for example. I'm sure that people do, just not the people I know. Only a marketing hack would download a couple of megabyte plug in to look at a soda company's web page.) SSL has a lot of problems (Verisign's pound of flesh, signatures on sites rather than documents, etc.), and those problems make a PGP based system attractive. But SSL's ubiquity (is that a word?) and the inherent kludginess of a PGP based plug-in make me think that the latter wouldn't have a snowball's chance in hell of catching on.