Yes, you need the whole arm. That's a DOT compliance rule. Think of sitting on the back of a bus, going over a bumpy road. Maybe not so much on the xB, but a number of vehicles drag air at the blade, they lift off the glass slightly, and between the two design flaws, wind up shooting back at other cars when they fail.
I don't know when this was mandated, but seeing a wiper blade fling off a car at 60+ mph is an 'Oh sh*t' moment for sure.
Good thing they are pretty cheap, and if you periodically clean your blades with the brown paper at gas station courtesy buckets, the blade will last for many years.
The retention nut does get rusty, you'll want to use care in removal, because you can't just replace the stud without rebuilding the rear wiper motor if that thing snaps or strips.
Some people do a wiper delete, use that hole to plop a backup cam. I'm sure you can search the site and find a few examples.
And you could buy the wiper assembly of anything you like that accepts the stud diameter and thread pitch. I haven't seen it on xBs, but others want either a bigger blade or that ability to just replace it with a universal cartridge. I'm not sure I would bother, or even recommend it. But if you want to be that rebel, there ya go. Don't blame me if something bad happens.