Use of existing infrastructure will be the preferred choice but they may use poles if there is no available underground duct,existing duct maybe full or blocked/damaged . Duct overlay may add significant cost and time and result in an area becoming unviable.
As will future damage caused by cheapness over quality build wise, (but the same costs are passed on to the consumer)
There's an aesthetic element too, especially in an area that doesn't already have any poles, that can cause homeowners to come over all NIMBY on the planning people.
I am pretty sure that my own fibre install stalled for 3 years because a pole that was supposed to be installed outside someone's house, never appeared because, I suspect, they opposed it. The planning notification went up to say it was happening, sat there for 6 months and then quietly disappeared again without a further word being said.
Fortunately, my own install is now moving forward (see other thread about YouFibre) using the mix of poles and ducts that my end of the streets already has, but I foresee some bumpy times ahead for the rest of the estate, much of which has no poles, no ducts and, according the cable map, dug-in cables throughout. Either the whole lot will have to be dug up for new ducts, or they're going to put poles up. And, knowing some of the people that live up there, I suspect that the cheaper option of poles is going to generate enough opposition as to cause a bit of a headache to those who would rather have better internet.
Apparently, YouFibre are heading up there next, so I guess it will be interesting to see which option they choose, and how it's accepted (or fought!) by the residents.