I used to occasionally wear a helmet, on and off really. This time last year, I came off my road bike at somewhere between 20 and 25 mph, sufficient impact with the tarmac to break 2 ribs (although I didn't know it at the time). Anyway, I carried on with my ride, got home (not realising I had cracked my head on the road too), and took my helmet off.
I was shocked to find the side of the helmet, seriously scuffed and one of the 'ribs' across the top of the helmet cracked. What it would have done to my head had I not been wearing it, I don't know but I do know it would have hurt a lot more than it did!! The only reason I had it on was that it was damn cold.
Needless to say, I ALWAYS wear a helmet now. I don't preach to anyone, it's individual choice but, if I was you, I'd probably get a new one and I'd make sure I wear it!!