I guess usually it's for looks and strength. Where I live there's never going to be a problem with sucking in hot air. I used stainless pipe and rubber elbows because it was an easy plug and play way to do it and I think it looks good. I also used some nice looking hose clamps instead of having to glue it so I can pull it apart easily any time. Plastic pipe can also crack if it's struck but that depends on what type of driving you're doing I guess.
But as they say: horses for courses.