My sense at this point is that they basically don't. My experience has been that while there have always been recruiters who were shady and/or flakey, until about 5 years ago a significant enough minority were at least borderline competent at the task they were nominally intended for (to wit: provide ballpark vetting of candidates + finessing the negotiation and relationship, generally). And up until then, they (also) seemed to be in it for the longer haul -- and to at least intuitively understand that referrals and repeat business actually matter, and hence, that one's reputation is extremely important (and not to be jeopardized by flakiness, shabby behavior generally).
That is: I never used to particularly like dealing with them... but there seemed to be about 15% or so of their lot who seemed to be basically honest, and if not particularly gifted at the vetting and relationship-building part, they did manage to create value once in a while. Also, there wasn't nearly the stigma against them (among both companies and developers) like there is now.
By now, though -- it seems this 15% has basically dried up. Now, it's not just fake chumminess (and the considerable slice out of our paychecks), and occasional lies we have to expect in dealing with them -- nowadays, they routinely lie, and routinely blow you off the minute they don't see a commission coming. They blatantly don't care about repeat business with you, or their reputation, at all.
There might be one or two out holdouts from the older, by no means "golden" more more pragmatic, business-oriented days of this profession -- but if there are, I don't see any significant chance that I might run into one.