Yes keys do exist for a reason. But he was a thief was he not? He didn't have the key. He was trying to break in. Having traps is very realistic. Making it a puzzle is very realistic. If it wasn't a puzzle, or a challenge, then there would be no reason to put the lock on the door in the first place. If a, lets say, "not so smart" guy could break into the vault then there would be no point in putting the lock on the door. The key would have the exact combination the picks needed to be in, the exact distance up or down they needed to go, a thief would have to figure it out for himself, and since its a treasury and they want to discourage people from breaking in, lethal traps would be a plus. If you heard on the thieves' graqevine that the last 3 people to try to break into the treasury died, then you would be very hesitant to try it for your self, unless you were very confident of your abilities. So, in conclusion, traps and puzzles are realistic, not having traps or not making it difficult to break in (aka puzzling) would just be stupid on the part of the treasury owners.
And to add a little more on realism and believability, i say sure make it realistic but only so much as it improves the game, if the realism makes the game frustrating then its unlikely you would continue getting games from that publisher. For example, Gothic 3, looks like a great game, to me at least, seems to be a lot like Elder Scrolls, however the developers, JoWood, did a terrible job optimizing it, i can play Oblivion on ultra high, but i cant play Gothic 3 as low as it can go, i get maybe 2 frames a second, so im never getting a game by JoWood publishers again. anyway, people have different "fuses" some people can stand more realism before the fuse ends and they blow up and break the disk in half, other people have shorter fuses, so its hard to balance realism against a fun game because everyone views it a little different.