It seems to me that the big issue is this: was "economy of effort" ALWAYS the guiding principle during the Neolithic, when people were using big stones in quite spectacular ways?
We can, I think, recognize three scales of complexity in megalithic monuments:
1. Standing stones -- either erected singly, or in groups including alignments and circles. It seems to me from my reading on many sites in this category, that the builders always used what was to hand -- big slabs or pillars from scree slopes, erratic boulders littered across the landscape, or rocks fallen off cliffs or smaller outcrops. There are hundreds if not thousands of such sites across the UK. In the ones that I am familiar with, I don't know one where rocks have been carried by the builders a long way from their places of origin. And indeed, why would they bother?
2. Cromlechs -- burial chambers etc where large stones were lifted and supported by pillars. There are many hundreds of these across the UK. I know quite a few of the Welsh ones -- and there isn't a single one where I have asked "I wonder where the stones have come from?" so without exception the stones have simply been picked up in the immediate vicinity. Indeed some of the capstones (as at Garn Turne) are so enormous that the location of the capstone is the thing that determines the location of the burial chamber. Find it, lever it up, support it, and then cover it with a mound........ Steve Burrow, in his book "The Tomb Builders" confirms that this was ALWAYS the rule with respect to the tombs built in Wales between 4000 BC and 3000 BC. Opportunism and utilitarianism in practice. In some cases (as at Newgrange) white quartz stones were valued, and were brought in from some distance away for the "facing" of the monument, but that was a relatively straightforward "cosmetic" matter, given the small size of the stones being carried.
3. Complex monuments involving the erection of pillars, the placing of lintels to create trilithons, and the working of stone in complex ways. Only one example -- Stonehenge! Built over many centuries, and passing through many different phases. But would the builders have abandoned all of the simple principles adopted by their contemporaries who were putting things up all over the UK? I doubt it. Why would they want to go off to West Wales to collect their stones, if stones were available within easy reach? If the stones from Preseli were sacred in some way, why were they not taken from Preseli and incorporated into megalithic structures elsewhere in the British Isles? And if they were token stones or ancestor stones invested with particular qualities, why were there no equally valuable "magic" stones to the north, east or south of the site?
Looking at all of the stones in the neighbourhood of the site (and let's remember that there are around 30 different rock types) the conclusion has to be that the builders of Stonehenge were following exactly the same rules as their cousins in other parts of the British Isles. I agree that they had greater imagination, aspirations and technical skills than other groups, but in the end they still had to make the best of whatever they had to hand. And of course, as I and many others have pointed out, they never did have enough stones to finish the job, and spent a lot of their time moving stones around from one setting to another before finally giving up and walking away.