As any (westernised) Chinese restaurant will tell you, the dish needs to have both. You need the sweet to be offset by the sour just as the sour needs to be balanced by the sweet. If it is too sweet, the dish is overpowered by the sugar that flavours it. It makes perfect sense. You need the sour to appreciate what would be far too sweet otherwise. Now before you think I am a prattling foodie (guilty) and this blog has devolved into the words of an amateur aspiring culinarist, this rule remains just as true for other categories. And, if I can be so bold, this defines the universal rule: you need to have balance.
I was watching Dexter and even if the main plot line is far from rainbows and unicorns it generally follows a relatively smooth and 'sweet' path. The adorably altruistic endeavours of Dexter Morgan's plight to rid the world of mass-murderers is tempered by his dark passenger and lust for death. This is why characters like LaGuerta exist. Without annoying characters that do their best to disrupt Dexter's harmless endeavours, you would no longer care as much about Dexter. You like Dexter because you hate LaGuerta (or at least I do). If there are no antagonists to hold your actions in check, it just becomes a run-of-the-mill episode of Ugly Betty. And no one would want that.