Surely the common sense approach to stock management is not to spent thousands of man hours creating huge ditch and bank structures which you then periodically refill with clean material and the re-dig the ditch. The common sense approach would be to gather up a bunch of brushwood e.g. gorse and enclose an area that you want to pen your animal in. This would keep the animals in and the predators out.
Common sense would dictate that a circle isn't the most practical of shapes to choose as a stock pen. It's very difficult to catch a beast within a circular structure, corners are far more practical.
Surely a common sense approach would indicate that megalithic structures are not practical structures for day to day use. They involved a massive investment of time and labour - much more time than, for example, was invested in the building of houses and other domestic structures.
Then there is the archaeological evidence. Henges were not maintained as domestic structures. They were cleaned, maintained, they contain burials and objects and been placed within them. Henges have orientations and relationships with other monuments both natural and man-made.
None of this is a quest for spirtituality in the 21st century it is based on observation, excavation and common sense.

Groan... just typed a long reply to your post fitz and then hit the 'paste' key instead of the 'copy' key - it's gone and I haven't got the energy to do it again tonight but will reply soon :-(

Yes but... Are you not still thinking of the big henges of Wessex? Sure - they are massive and would have involved many people working over a longish time period. Yes they do reveal evidence of dwellings within and Marden, Durrington and Avebury are surely now seen as mulit-functional (doesn't Burl discount them as henges because of their size?). I accept all of that, but the point I'm making is just to question if ALL ring-ditch structures that we currently call "henges" were built to the same purpose.
I draw no conclusions whatsoever. I simply post a question.
Can anyone briefly summarise the evidence that proves that ALL henges (of whatever size) were constructed for ritual purpose alone?

Hi fitz. Since my last post most of what I was going to say has now been said by others. Just a couple of points though if I may. You say that, "Surely the common sense approach to stock management is not to spend thousands of man-hours creating huge ditch and bank structures..."
I've argued this before but surely the amount of effort one puts into building a structure is dictated by how long one plans to stick around - if it's just for a season then sure, a bunch of brushwood will probably do the trick. But a more settled community is going to look towards a more substantial structure. For example, why continuously build a bridge from perishable materials (that might be quick to construct initially but will only last a year or two) when a stone bridge is going to pay off in the long run (even though the stone bridge will take more man-hours initially it <i>is</i> likely to last for many more years).
You say that, "It's very difficult to catch a beast within a circular structure, corners are far more practical." Yep, can't disagree with you there but who said anything about catching a beast within a corral? To my mind a corral is a place where livestock are given shelter (overnight, or perhaps when there is danger) and then herded out to pasture when it's safe to do so.
You also say that, "Henges have orientations and relationships with other monuments both natural and man-made. None of this is a quest for spirituality in the 21st century it is based on observation, excavation and common sense." Yep, agree with you there as well but I think I'd insert 'many' - ie, 'Many henges have orientations...' and although I agree with you generally I must disagree with you when you say, "None of this is a quest for spirituality in the 21st century it is based on observation, excavation and common sense."
The quest for 'spirituality' in things like stone circles, wells, ley lines, crystals, vanished civilizations, etc etc is a burgeoning quest in the 21st century and a visit to any good bookshop or the internet will confirm that. I do not wish to belittle that quest but I also believe we should not be afraid to say (as others have said here and elsewhere) that some of the things we hold dear as the remains of our 'spiritual' past may actually have been purely utilitarian in function - that does not make them any less interesting nor any less worthy of our attention :-)