Of those classes that have multiple DPS specs, one is usually better than the other, particularly for the pure DPS classes. But the top spec for pure DPS classes rotates all the time, and rarely is one of their specs the worst in both PVE and PVP for more than 1 tier/season in a row. Even hybrids with multiple DPS specs usually have periods where one is better than another, and they flip flop all the time. For warriors, arms is generally second tier to fury in PVE with the occasional exception for AOE, but arms is almost always a top spec in PVP.
However, DK's seem to break from this trend. Frost is always outclassed by unholy in both aspects of the game. I can't think of a single tier going back to 4.0 where frost was a superior PVE spec. More often than not it is bad enough that guilds don't even want it in the raid. This might be tolerable if it was significantly better than unholy in PVE, but this has never happened either (the current PVP season is the only one in memory where frost wasn't totally outclassed by unholy) and right now frost is probably in the weakest position I can remember, even worse than ToT. My question is why? Why is this spec consistently shafted by Blizzard? Certainly other specs like enhancement have had tiers where they weren't great, but I don't think any other spec has been at the bottom of both PVE and PVP performance as consistently as has frost.