I would have to say, it depends. (for me) If I was buying a highly priced show/jumper/performance horse, then definitely yes.
If I was buying a horse I was just going to hack around on, then I feel that I have enough experience (personally) to spot a problem and if not, than that is a risk that I am willing to take. I also take along another experienced person or persons so that they may spot something I might miss. (nobodies perfect) So far for me, I've never had a problem. I will say that if I were to have any doubts I would get one.
That being said, if you don't have lots of experience then I definitely recommend a prepurchase exam.
As a seller, I NEVER have a problem with a buyer wanting a prepurchase exam. I once had a vet (that I had never met before) tell a buyer that if she didn't buy the horse then he was going to!