nope. Not vital. Unless they have yosemite sam on them.

I’ve ignored much more important parts of my car falling off.

Unless you spend time “muddin”, you’re fine.

Probably less drag… fractionally better gas mileage.

On some cars they keep mud etc from getting stuck inside the fenders or whatever. So if you pull it off and there’s a big hole under it then yeah you should replace it before a family of racoons moves in. Otherwise just leave it off