"Every age gets the Stonehenge it wants" (or something similar)
Its "what it deserves" I think. Translated as "what the cloddish unthinking voter wants" maybe. (Though we must remember we're a tiny anorak minority here!)

"It's hard to believe, though, that a bank and ditch project the scale of Avebury would be undertaken without having a clear purpose for each of its components."
Well, all I'm saying is if you asked my little grandson why there's a ditch round his sandcastle he'd say "that's where i got the effing sand from yer prat!". (We're a rough family.)

"so what are the other possibilities?"
Well, we know probably they thought the centre was special and that they built a ring ditch round it, but we don't know the crucial bit, how wide was the special bit? If it included the ring then the ring could be a screen but if it didn't then the ring could be a platform...

"In some ways I hope we never do find out what Avebury was used for"
Cobblers, if you found out you'd be delighted and rush off to arrange a publishing deal I reckon! Monumental about Pigs.