Some long-term job listings, especially the vague ones, are dummies put there by recruitment firms who are keen to get you on their books.
Edit: There's probably a book waiting to be written on the dodgy tactics of recruitment firms. Mind you, I became a bit less harsh on individual recruiters when I was in an office that shared a floor of a building with a recruitment firm and got to see how they treated their staff. They were too cheap to hire meeting rooms when they were doing staff reviews so we regularly had people getting complete bollockings in the shared kitchen area - which isn't much fun to witness and a pretty ghastly way to treat your own staff.
http://www.amazon.co.uk/Rip-off-Scandalous-Management-Consul...
I'd like to read an equivalent for recruiters and as more people encounter recruiters than management consultants I expect the audience would be much larger!
Many companies have more or less phantom job listings on their websites and the free boards because whilst they haven't got any positions that need filling, they can afford to create role(s) for the right candidates. And as you've said, some probably can't afford that yet but at least they look like they're growing and get the resumes of people interested in what they might be able to offer in future.
Larger companies often have job ads up for positions they expect to fill internally, because policy requires they advertise it externally.
Perhaps too many people are now wise to this? And so recruitment firms have had to arrange for articles (such as this one) to muddy the waters ;)