I have worked on elance for years (and odesk too, now they have both merged and elance is already starting to withhold contractor payments for a minimum of 3-5 days, I've heard reports of up to 3 weeks to get paid now), and like you said there are several reasons. Some people just post job ads to see what they will get back, some of the people posting are contractors looking to see what other contractors are bidding, some are people writing blog articles about it, or others are just scammers looking to pump people for information then try to do the project themselves, or have their in-house team do it once they have some info on a solution/approach.
I've had many many clients waste my time getting detailed specs, solutions, etc.. and then never hire me or anyone else for the job. I'm talking insisting on a "phone interview" then keeping me on the phone for hours asking every little detail about the project and how I would handle it.
I've learned to not give out so much information, so it's better now, but I'm getting off track. Your other question was, can you assume after 30 days that the job is dead. Simple answer, NO! I've had people hit me up 3, 6, even a year after I bid the job and end up hiring me and making some good money out of it. However, it is RARE, and maybe only happens once or twice a year to me. And I bid a lot of jobs. A lot.
I have a policy (stated in my profile under payment terms or whatever), that my quotes are only good for 30 days and after that time, a new quote must be obtained before hire. This is because if someone comes back 6 months later and I was short on cash at the time of the bid and only bid at like 30/hr, then 6 months later I'm doing better clocking 6k a week, I'm not interested in chincy 30/hr work anymore, so I point them to the clause in my payment terms to handle that situation. They usually bail, sometimes they are willing to pay more.
IMO, if a job has not contacted you within 1 or 2 weeks, forget about it. I usually withdraw my application. If someone is going to waste my time and not even bother to respond, then I don't want a response, move on to better clients.
Elance does not charge the client a dime. They can post as many free jobs as they want and there is no penalty for not hiring. Another thing that happens is a lot of these guys will try to get you on Skype and move off the platform. Something that I'm sure happens a lot on there. I don't do it because there is only one reason someone wants you to move off platform, so they can burn you.
If someone pays you through Paypal, even if its like 20,000 dollars over the course of a year, at any time, they can call their bank and reverse the charges and Paypal does not protect you. You will have a -20,000 balance on Paypal. It's happened to me more than once, not for that much, but basically any money you get through Paypal is not really yours, ever. They can always take it back, this is what I've been told by Paypal account specialist. So I'd rather have elance do the transaction, as they have never ripped me off (for a payment, they rip me off constantly on fees and other redic stuff).
Anyway, the two have merged now and neither is worth working for. The overseas contractors really threw salt in that game, so for anyone looking for decent wages, it's back to the old 9-5. Though I have been thinking of trying fivverr now that they upped their limit from 5 dollars to 500 dollars a job.
Anyway, kind of all over the place on this... I hope I sort of answered your question.