I've been thinking about this one since I read about how strongly people feel regarding the Star Wars Expanded Universe. Katharine Trendacosta naturally makes a lot of good points - on fandom exposure, on writing, and particularly on reading. Reading actually is a very important part of fandom participation as Katharine explains - and for many sci-fi franchises (such as Ender's Game prior to the movie) it still is the primary means of directly participating with that franchise.
I feel like reading, for me, is particularly important. Katharine talks about how important EU books are for younger readers in particular. I don't think the feeling of reading the latest adventures from Luke or The Doctor really changes for older readers, either. Or whatever those stories might be - even if they're about a bunch of teens who, say, fall in love or fall in love or fall in love or...er, you get the point.
Reading As Expression in of itself
A lot of talk is made of the craft of writing as allowing one to "express" oneself. I'd argue - actually, what I discovered after looking over my personal reading selection is that the act of reading itself becomes an expressive art. Writing is, in essence, a two-way street: the writer shares his or her experiences (including ones that the author entirely makes up from whole cloth) with the reader. The act of reading therefore becomes one half of an expressive experience - but still an expressive experience.
The best authors are able to carry over their own expressions and feelings onto the reader - when the reader laughs or cries over a good book. Being able to vicariously experience other's feelings through books expands one's ability to self-express. Think of it this way: you can pick and choose what you wish to feel, and your decisions become their own message. A lot can probably be said about a person who, say, reads a lot of David Levithan or Ann Brashares.
Reading as Interaction
In an odd way, reading actually involves a ton of human interaction. You're communicating with the author, albeit very, very indirectly. You're also more directly interacting with the characters in that book, as fictional (or distant) as they may be.
Go back to where Katharine talks about how many sci-fi writers were inspired by the larger fandoms they belong to. As she says, they were inspired by those franchises. And they themselves managed to get a fandom following that were in turn inspired by them. That's actually some pretty powerful interaction right there.