Are important. They keep your mast(s) up. If they fail it can spoil your whole day.
These are the chainplates from my last boat, a Downeaster 38, after being removed by the new owners. I had mentioned that the chainplates might need replacing so they were pretty understanding (thanks Jack and Susan). Plates were approx 1/4inch by 2inch. I just didn't know how soon they needed replacing. I went so far as having all of them x-rayed in Trinidad about two year previously and they were given the all clear.
So, hopefully, I have now learned my lesson. If in ANY doubt change them out.