I know that for me with snowboarding after getting out of my beginner stage I don't really fall a whole lot and if I do fall it's really pretty controlled. So it's definitely really easy, especially when you gain more confidence, to decide to not wear a helmet. Last season I didn't even have a helmet.
On the other hand, I work on an ambulance. I've seen my fair share of concussions and their consequences. And to be honest most of the time it ends just fine with no real harm done. But I've seen people paralyzed from spinal injuries and people die from head trauma.
The last day of last season I took a very hard fall when helping my girlfriend learn to snowboard, I let her go ahead of me, watched her, then made my way down with not a care in the world. It was basically a bunny hill why should I be careful right? Long story short, hit a bump I didn't think was there and ended up hitting my head about 5 times on my way down. took me almost 30 minutes to get my bearings straight before I finally wobbled down to my truck. This year I'm using my kayaking helmet, just so I have something. definitely not spending a hundred bucks just for another helmet.
So no, if you are doing easier runs then you don't NEED one. Of course you could get seriously injured snowboarding at any level, it happens all the time. Ultimately it's your life and it's your decision, better safe than sorry? or live with the consequences?