Should I wear a helmet?

I enjoy biking and Im going on the trinity river trail in Fort Worth
this weekend. (70 mile round trip) Im trying to keep costs low and one
of the last things I would expect to get would be a helmet. I dont
intend to fall off and hit my head. Even if i do....would it help? has
anyone had any real experience with a helmet in a fall? The only
anecdote I can provide is one time I was backpacking at Dinosaur
Valley. I was packing up my tent in the morning and heard a sound....a
couple of mountain bikers coming by. They stopped and chatted with me
a minute or two beofre riding on. After several hours of hiking i made
it back to the road and my truck. There was a truck parked next to me.
As I was putting my gear up along came those 2 bikers. One of them was
carrying his helmet and had blood on his face. He told me that he was
going down a steep hill, hit a root and got thrown from his bike and
his head hit a boulder and split his helmet in half. He said you gotta
throw it away and get a new one if you even get a small bump. From the
looks of it, the blow would have surely cracked his skull open because
it was split wide. So even after seeing that...I wonder if i should
spend the money on a helmet. I dont have much to spend and it seems
uneccesary because I wont be doind the kind of stuff like that which
gets you into those situations.....but since I also go up to the store
and back you can never be sure.

How many other people ride without a helmet...and how many people who
do wear helmets find that its been a lifesaver? Also, have you worn,
been in a fall and still got hurt where it didnt provide any help?

any input would be appreciated.