As many have already said, it depends on the situation.
As for "something-is-better-than-nothing" situation in some RPGs, I will usually pick one of the helmets which meets a decent blend of style of functionality. If the best item is utterly hideous, I'm probably going to go for a passable one (statistically speaking) which at least looks decent; I'm not an obsessive min/maxer. Just as an example, I'll wear the visor in ME2 (small enough to barely be there) over some of statistically better helmets (the Kestrel helmet? No, just no).
---
One of the more common features in RPGs lately is the option to toggle on or off helmets, you'll get the statistical benefit but can choose whether or not it will show up visually; an interesting choice, but still dependent on the situation (in my opinion).
Here, it's a matter of how good the helmet looks. If a helmet looks good on a character, why not leave the visual on? Other times wearing a helmet just makes sense; on a tank-type character, the helmet (usually) makes them look tougher.
Again, I tend to go for a mix of style & function (and I'm often willing to ignore min/maxing to do it). Even when turning off helmets is an option, I'm not above wearing a helmet which has an appearance I like; as opposed to a better one which looks awful (though significant stat gaps will make me consider the alternative).
---
Anyhow, it's all dependent on the situation. The setting, the character's role, the helmet itself, and even just how I'm feeling at the time. There's no one clear-cut answer to all of this.