> While the ethernet cables they give out can likely do 10Gbit (but definitely not 25Gbit) very few people have 10Gbit-capable ethernet or wifi chipsets and there is no way they will actually be able to routinely transmit data at this speed.

Bit of future proofing, the fibre cables will be in the ground for 10 years and who knows whether consumer devices can routinely do 10G by then. The cost is dominated by the price of digging up the roads, not by sticking a few extra strands in the ducts.