Lots of good thoughts around here.
I personally go with Hassat Hunter here. I would myself like to not have any boss at all. There are other ways to do difficulty peak.
On the other hand, I'm also with Dr Koin saying that bosses are pretty often the end of a part/zone/chapter. Which is great. You know as a player, when killing a boss that you achieved something in the game, progression, or something like this.
The bosses I like in game usually is, like Lacrymas said, the ones who test the stuff you learned in the zone.
For me that was not the case in D:OS, it was mostly big bad dude/dudette who where here to punch you in the face, sometimes cool, sometimes cheap. It's ok to some extend with the game because there's no way to exactly know how the player will spec their characters. In D:OS you can know, to some extend, which skill the player might have but that's still a lot variables.
It's not like Super Mario Bros or The Legend of Zelda where this is the most simple form of Rational Game Design. In this particular level you give this power to the player, he learn all the way through the level how to use it and then the boss is there to challenge him.
Designing boss is a great challenge. Especially designing FUN bosses. Because designing nasty annoying boss is pretty easy. But in the end, they are not just fun. We could go to Darksoul on that matter, bosses destroying you, that are fun. I guess that's another subject, haha.
