I once asked a tailor, who shall remain nameless, if he could make a custom suit with an unlined or half-lined jacket. He looked at me like I was crazy, and actually asked "why would you want an unlined jacket?" He gave me this spiel about all the benefits of lining. I thought at the time that he was an idiot; now I think he was probably just lazy. This guy runs a major tailoring establishment in Bellevue, WA (very affluent), which is why it surprised me so much. If it had been a no name shop I would have expected it. I personally like lined pants, they don't catch on my legs when I sit down, they're a little easier to put on and take off, etc. For warm weather pants though, it's a tossup.