If you are going from IAH to IST, the arrival time is like 5pm. Then going out is not a good option as it's getting dark unless it's summer that you get more daylight.
The IAH flight leaves at 13ish which means that your BCN flight is either a midnight departure or early morning departure which doesn't give you much time to go out anyway.
Like the others said, go for it if you need the miles and if you have lounge access