Diabolic tutor lets u get any card from your library get ring of three wishes and keep gettin more rings
Or
{demonic tutor}
{reverberate}
The ability to search for a particular card in your whole library is a neat thing to do, but the benefit of having many tutors (and rings) in a deck is kind of cancelled out by virtue of them taking up the slots in your deck that ought to be used for the very cards you'd use the tutors to find.
The way I see it, tutors (because they don't win games themselves) are moves of desperation. And you ought to have built a deck that doesn't find you desperate that often, ya know?
You can also think of it as hedging for your win condition. If you have a 4 card combo, that's 16 cards. Demonic tutor can replace any of those cards and now you have 20 cards toward your combo. When you also take into consideration that you need 4 unique cards from the original 16, but getting a hand with 4 tutors will allow you to combo, their value just went up.
I prefer to think of these types of cards as building a 56 card deck instead of a 60 card deck.
This isn't quite true on account of the cost it adds 4 cmc to the list of getting your combo out, and slows you down a turn or more.
A favourite of mine for this is {jar of eyeballs} in sac or weenie decks, but popping it is still a desperation move, as already said about the tutor.
The better way is to fish for combos with a well balanced deck and scry or card advantage.
I even rate sac-lands above this because it thins your deck for you without always slowing you down much. A two colour deck with 4 evolving wilds and 4 teramorphic expanse effectively is like having a 52 card deck with 8 gates in it. (not exactly but i hope you get my point?) :)