It's often a bit of a farce. I've never seen the point of having it, given that the "king of the mountains" is usually by definition, the winner of the general classification.
Especially in recent years with the reduction in ITT kms.
Well, the way racing has changed has been the major impact on that. And Richard Virenque pioneering the "king of the breakaways" method of winning it. Nowadays a one-dimensional specialist climber such as those that fought for the race by winning the GPM has no reason not to believe themselves a contender for the GC win, whereas back in the day climbers could only win a GT if they were truly exceptional, like Charly Gaul, José Manuel Fuente, Lucien van Impe or Lucho Herrera.
The GPM was created as a means to give people like Vicente Trueba a reason to compete. The pint-sized Spaniard was always the best over the mountains of the Tour in the 1930s, but he would lose hours in gruelling 3-400km flat stages because of his lack of top end power and he would suffer on the bad roads of the time. However, the mountains were becoming the big attraction that they are now, and so Trueba was a very popular rider who was nevertheless a GC and points competition irrelevance.
For the next half a century or so, in Spain the GPM was considered the second most important thing to the GC; it was better to finish 10th and be KOM than to finish 2nd and not have anything to show for it. This was largely the product of a less developed cycling scene in Spain, and partly to do with the regions that were most supportive of cycling - País Vasco, Navarra, Galicia, Asturias, Catalunya - all being mountainous regions and generating climbing specialists. The best example of the old fashioned GPM is Julio Jiménez - El Relojero only managed one podium in a Grand Tour but won several GPMs as part of his bid to compete. People like Fuente and Herrera won the KOM because they would lose a lot of time in 100km+ of ITT in those days, and have to fight back in the mountains with attacks from deep, the kind of move that isn't really possible more than once or twice a race in modern cycling, rendering the big saw-toothed stages days for the break to accumulate its mountain points.
You could possibly argue that Virenque's method was a development of José Luís Laguía's, in much the same way as US Postal's method was a refinement of Banesto's; in the early 80s, the Colombians had come and stolen people's hearts in the Tour, and the Spanish no longer held sole preserve in the ranks of nations noted for wispy, unreliable climbers as they had done previously; as a result the top Spanish escaladores of the early 80s are not the household names that their 70s precursors had been. Nevertheless, Laguía would not be the first name you'd think of for Spanish climbers in the 80s, because surely people like Ángel Arroyo, Alberto Fernández, Pedro Delgado would come to mind before Laguía, but he's the record holder for Vuelta GPM. Virenque refining a method of GPM collection which was not part of a genuine attempt to win the race or even the stage but just to accumulate points, coupled with some terrible stage designs in the late 2000s that meant huge points were available on stages like Tarbes 2009 and Pau 2010 where everybody knew the breakaway would collect all the points (plus the removal of time bonuses at stage finishes meaning the GC guns let the break fight the stage in the mountains much more often) and further moved the GPM towards being almost a surrogate for the metas volantes.
The restructuring of the classification in 2011 has worked to some extent in terms of the calibre of rider winning the GPM, but the design of the course thus far means that apart from Anthony Pérez who has obviously now crashed out, nobody has really set their intentions on winning the GPM other than Cosnefroy, so nobody is contesting him for it. Once people have dropped out of the GC mix, they may reappraise their goals and target it, like Bardet or Majka in previous years, but for the moment nobody serious is considering it or even rumbling over the mountains behind Cosnefroy in case it's of value later (this is what Charteau was doing in 2010 when Pineau was collecting summits in pre-agreed deals early in the race, for exxample), and some of the people who've collected a few points from being in breaks like this might put it to use later in the race if they're in position to target it. But there aren't many people who go out pre-race to target the GPM like David Moncoutié used to at the Vuelta or Leonardo Piepoli did at the Giro.