If you are considering creating a web-based archive then I *do* suggest you use blogger, MT or another open blogging tool. LJ is just plain evil. Not because it is ugly (though it is) but because it encourages insularity in its users. I've noticed LJers are far more likely to post links to other LJers than to external links.
Yow! I guess the reason LJers are more often likely to post links to other LJers is because of the whole friends list thing. It makes it easy to add other LJ users, but not so much other blog users (and I see Mags has a blog on blogspot. hmm!), though now that they've done away with the syndication quota that might become a little less true. Though it's still the case that only paid users can make new syndicated feeds, so... I dunno.
Does LJ stop its users reading other web sites? No. Having a LJ hasn't stopped me from reading other blogs, even when they don't provide an RSS feed so I can syndicate them to LJ and put them on my friends list (e.g. Doug Daulton - check out his entry on his introduction to computers and their subsequent growth).
LJ's main advantages over other blogging software is that it's free and extremely easy to set up and use. Without that, there are a lot of cool people blogging/LJing who I believe would never have done so otherwise.
Though 3 million of them is probably overkill. :)