Yep, agree. I just got a vest the other day... hadn't had one previously, but I'm wanting to get a little more seriously into eventing and if I want to move up the grades any more I have to have one as they are compulsory at 2'7" level and above where I am.
But, I know people who have had fairly minor falls in their XC vests, and kept them, and then had very very nasty injuries from subsequent falls when the same fall with a brand new pristine vest would have prevented most of said injuries.
A vest WILL NOT protect against twisting injuries, or spinal injuries. It also will not protect much against crushing injuries. However, they are BRILLIANT for impact injuries, and abrasions and lacerations.