"Could be, but depending on which side of the ditch the spoil is deposited reflects something very important; if it's on the inside of the ditch it surely suggests a defensive purpose. If the spoil is on the outside it suggests something quite different - certainly not defensive."
My point is that those are modern perceptions of what is there. They may be well founded but equally they may not. There is a (reasonable) certainty there was an intention to create a bank. There is no equivalent certainty that either the ditch or its position was of equal importance. If it had been created outside the bank (again, possibly for the mundane reason that they needed to get material for the bank) then no doubt we would be theorising about the ritual significance of having the ditch on the outside. Had they got their spoil from a mile away we'd be theorising about the ritual significance of the Beckhampton Hole. ;)
In essence, my point is : don't think ditch think quarry, and it all looks different... (Maybe).