Have you ever tried to explain to someone what a cliffhanger is? I suspect they became popular when Dickens, Trollope and co were writing weekly instalments and there had to be something exciting and dangerous so that people would remember and buy the newspaper or magazine the following week. Classical scholars would cite Scheherazade, who managed to save her life night after night by telling the Shah of Persia stories and always ending on a cliffhanger. Try that for a thousand and one nights and see how you fare.
You could argue that cliffhangers exploit the reader. The author chooses to tie the heroine to the railway tracks and leaves her there with the distant hoot of a train in the distance, so of course the reader gets agitated. If only someone would find and free the woman then we'd all relax - and forget to buy the magazine next week. As it is we spend the entire week in a frenzy of will she die - or will she be saved?
Too many cliffhangers in one story do exploit the reader - or they annoy and irritate at the very least. But a good, gripping cliffhanger is worth it's weight in gold, and it doesn't have to be in a thriller, though the one I'm reading at the moment happens to be one of Dick Francis's classics - Come to Grief. At the end of Chapter Twelve, the hero is tied to a chair, and has watched his prosthetic arm destroyed by a one-time friend - and you just know that there is worse to come for Sid Halley.
So the essence of a good cliffhanger is the build up, I think. First of all, there has to be empathy for the victim, otherwise the reader doesn't care enough to worry. Then there must be a slow tightening of the screw, tension has to rise somehow, and then - only then, can the author bring the cliffhanger to its proper threat. And escape must seem impossible.