I'm a fairly regular "growbag" wearer - sometimes for professional reasons, sometimes because I just choose to. Either the full suit, or just a jacket.
Why? Looks reasonably smart in my own eyes, pockets in the right places, keeps my street clothes clean of grease and mud, and in extremis - yes Nomex is extremely flame resistant, worn correctly.
Can't say I have a problem with it, would never say that somebody *should*, save in particular environments. Test flying and particularly first flights I almost invariably do, and that's an appropriate environment as an aeroplane that's never flown before has an above average risk of stuff going wrong. Similarly in work research aircraft full of semi-certified equipment with lots of wiggly amps all over the place - I value the flame retardance and have on occasion insisted that other people wear one. Also in an open cockpit, tends to let less drafts in than other forms of clothing.
Not necessary on a couple of hours trip in a 4 seat spamcan, and looks a bit stupid in the flying club bar - but can't say it worries me to see people wearing one. Just make sure that any badges on it you earned the right to wear !
G