My reason for buying a diagnostic kit: (feel free to call me crazy)
Every year, the students at our school have this debate. We are "expected" to have an oto/ophtho kit for our physical diagnosis class, and for our neuro clerkship. In addition, we have a couple of old school medicine preceptors who insist you have it for the medicine clerkship (if you are assigned to them).
What we always hear from upperclassmen is - oh you won't really need one, and if you do, just borrow it from one of your friends.
My problem with this is, I'm basically saying to my friends "I don't want to pay $500, but I think you should, and then you should let me borrow it whenever I need to." That's kind of inconsiderate, IMO.
So I bought mine. I used it 8-10 times for physical diagnosis. It gathered a bunch of dust. Then during 3rd year, every once in a while we'd get some random complaint about ear and/or eye problems and need to take a look. At our hospital at least, finding a scope on the inpatient side is next to impossible. So I started carrying my diagnostic kit in my bag - no, it doesn't get used often, but my interns have really appreciated that I can dig up a scope for them and save them the effort of doing it themselves.
And, in the meantime, I've had three friends (who didn't think it was worth the money last year) ask if they could borrow my set for 5 weeks for their neuro rotation. I told them (nicely) no way.