From memory the last time I was actively looking they call you all the time when they want to put you forward to something and then drop you like a hot potato once you don't get the job! I can understand that, it's a numbers game to them and they need their commissions but it can still be frustrating. It's like they have you in mind for that one role, if you don't get it they look elsewhere rather than seeing what else they have that could be of interest.
This is a major frustration for a lot of people and I think there are a number of factors why this happens. For me its a combination of targets the recruiter must hit, Contingency based recruitment where the recruiter is competing against a number of other agencies on a commission only basis doesn't help either as they are looking for the "perfect" candidate as that's their best chance of earning commission, lack of proper internal training and the idiots the industry attracts.
The recruiter also has to weigh up if its worth their time actively promoting a candidate to their list of clients or work on the vacancies they have. Depending on your experience and sector, recruiters will be different I would imagine.
I can only really say from my niche sectors that I cover I can't afford to speak with a candidate then drop him as it were as it's too incestuous and we would cross paths again down the line. There are recruiters who are very vacancy driven then there are others who are very candidate driven where they would seek to deal with a candidate who may just be considering a career move but not actively applying to roles.
Experienced individuals looking for a role if your considering using an agency I would recommend doing a little research on which agencies would be best to represent you.
A boutique agency may be better suited as they may be in a better position to approach a range of companies within your sector as they would be more prepared to negotiate on a fee their end.