I will defer to printers' stated needs to save ink or promote faster drying: Many factors contribute to such conclusions and will vary with printing method and ink costs. Juts to give two examples, newspaper printing on a coldest press and flexo printing on poly bags with UV-curing inks are so different in printing problems as well as ink cost that it would not be possible to make any blanket statement that would apply equally to both.
I can address a few specific points raised about ink savings solutions, however. Gordo is correct (as he usually is) with regard to most standard press profiles today: They are built with GCR, though the amount used is modest and intended to help with gray stability not ink savings. He is also correct that dedicated ink savings solutions replace the CMY neutral component far more aggressively than conventional GCR. Not only more aggressively but more selectively, replacing in shadows far, far more. There may also be a provision for protecting delicate near-neutrals and a tolerance adjustment that allows the user to balance the ink savings with colorirmetric accuracy, since the most aggressive settings will feather farther into chromatic colors. GCR will, even when set very strong, save only modest amounts of ink overall, as it favors accuracy above all and does not dispropotionately affect dark areas, where the greatest ink reductions can be exploited.
To give an example, I converted a test form heavy with images in ColorLogic's ZePrA color server with both 100% GCR and CL's SaveInk set to maximum; TAC was 280% in both cases. While it was obvious that the GCR conversion reduced CMY inks (which does reduce cost), the overall ink reduction was less than 3%, while with SaveInk the reduction was 24%, with a tolerance set to 2.0 dE00. SaveInk's CMY separation is startlingly sparse, yet the composite images look just fine. This solution is particularly adept, building ink-saving device links on the fly with automatic (or manual) purity exceptions, protection of flesh tones, and a tolerance setting.
There are many ink savings solutions on the market, and all will save a lot of ink. ColorLogic's ZePrA/CoPrA SaveInk combo is quite "safe," though I would be irresponsible if I did not caution care with any of these systems. AS Gordo said, sharply increased use of black ink places a premium on the quality of the black, and not just is physical consistency: It must be neutral in color, or the gray replacement algorithm will be adversely affected; a strongly colored black cannot be used to replace CMY grays without addition of more chromatic inks to neutralize the color cast. This is not the place to be using a "barrel" black mixed from workoff inks. In addition, sparse CMY separations may affect press stability, as there may not be enough ink in the train to allow sensitive adjustments of the keys. And as Gordo mentioned, there is the greater visibility of moiré (obviously alleviated with FM screening). Used with care, ink savings systems can be a powerful tool for improving print quality and boosting the bottom line.
