Angie, I've only had pre-purchase exams done 2 out of 3 horse purchases, so my exp is limited, but FWIW thought it still might help...
Vet only recommended xrays for mine when he found a lump on one of the horse's hocks (turned out to be nothing). Although I was asked if I wanted xrays as part of the exam, so it must be a common 'option'....?
When my vet was purchasing a horse for his daughter, he xrayed all legs, if that is a good example...
In my (novice) opinion, I think it depends too on what you are expecting out of the horse and what its background is. If you want the horse to be high performance (in whatever discipline), then to minimize your unknowns with xrays is beneficial. But if you're just wanting to hack around, may not be worth it. Also, if you're buying a young horse barely ridden or not broken out, you may not want xrays to determine wear and tear on the joints. But if you're looking at a hrose with 1000's of miles, or ex-racing TB, you may want xrays. Although there are always exceptions, yes? The one horse of mine that didn't get the pre-purchase exam that was bought barely used, turned out to have advanced arthritis in the hocks and stifle....so I'm like you in wondering if the exam would've caught it....:)
Good luck!
Cindy
ps - oh yeah, there was no blood work done for mine....