Firstly, I've no idea how FTTH is planned so the rest is just my own thoughts.
I would imagine each install is planned on an individual basis. As with CU/ALI circuits, it all depends on what the circumstances are like in the here and now, as opposed to yesteryear when the 'Direct in ground' armoured cable was lain. This type of cabling seemed to be prevalent in the 60's/70's' so a lot may have been done to the premises or surrounding area that could prevent replacement underground duct work from taking place ??
On a costing basis, on an average length house frontage ........ it will always be cheaper to go for the laying of duct, and I think protocols are to keep the install as near to the original work as possible.
That said, if as mooted above, laying of duct will take the costings through the roof (or past the Planners budget
), then having a pole erected would be the most frugal step forward.
Some members on here would probably insist that OR erect a pole outside every house, and run 12 separate ducts to each premises ......... just in case. <joke>.