I would of thought BT would take the less scenic route to save them money
They're more likely to save money by utilising existing cable ducts (even if it involves taking the pretty roite) as copper is much cheaper than digging trenches and installing new ducting.
As an example, my direct distance to the exchange is 2.65km. The road route (the main road which goes past my house also goes past the exchange) is 3.21km - this would be the most sensible router for a cable as this road has cabinets all along it.
My actual cable length is 5.23km (6.94km if you believe BT's dodgy database).
So; for me (and the other few hundred homes on this estate), BT have used at least 2km more copper than they would have needed by following the main road - presumably to save on installing new ducts (maybe the existing ones were full).
As for underground wires - fantastic! They are far more immune to night time interference as overhead wires (most of which is caused by long distance AM radio stations operating on the same frequencies as ADSL).
I have underground wires (all the way back to the exchange) and my 5.23km line can hold sync for weeks with a target margin on 3dB.
ADSL2+ with BE
DL Sync around 4.8Mbps
UL Sync 1088kbps
DG834GT with DGTeam firmware