As subbes said,
The community base, the friends, colleagues, [etc.] I've got here is the one main selling point of LJ. And it's the hardest to replicate anywhere else.
(Yes, I have a GJ, IJ and a Blurty account, plus a Vox. I have an RSS feed aggregator. They're not the same. RSS feeds are public, LJ f-list includes custom securities.)
Or, as someone else put it, "the fact that everybody and their pet rock has a LiveJournal" is part of the appeal. That kind of goes away if the place is a less desirable place for "freeloaders", because it'll no longer be the place with all my friends or with my community or with interesting and diverse content. (Though that depends to some extent on how people feel about the ads.)
There's five year's worth of evidence that proves that LJ users react worse when changes are snuck past them than if you warn them in advance.
which is the point I forget to make when people say "Ooh, a company trying to make money rather than go out of business; film at 11".
No small part of the reaction is due to how this business change was communicated (or rather, not communicated); if things had been, "we needs moar munny, u can has ads now kthxbye" people would still have been miffed, but differently, and most likely less.