Including a picture of my

It depends.

They did that in the old days because it was relatively cheap. It's not cheap anymore.

I did it on my recent house addition. I think I spent roughly $10 a linear foot for materials. Framer installed it so I don't know that cost and I had to apply a clear finish. The biggest challenge was trimming out between the rafter tails and providing attic ventilation. I lived in a house once that had open rafter tails and all it had was screen in some of the openings. The siding has to extend further up the wall, but you don't have to buy a soffit.

My house has open eaves. It was engineered with the trusses

If you spray foam the underside of the roof and make the attic conditioned space you don't have to worry about venting the attic. Its a fairly common solution these days.

Bruce