Quoting by the sq ft has been and continues to be a standard metric for some trades in home building. However, a subcontractor or trades person is free to bid the job any way they want to. Bidding is different from quoting, so I would tell a sub bid it the way you want to, count your material & labor, calculate time, costs, etc, but quote it by the sq ft. Some repetitive jobs do lend themselves to quoting by the sq ft. I know large subs who do production homes and the houses vary so little that they can actually bid and quote the job using sq ft pricing.