There is some flexibility in the roles of each class in lotro, but you are right, it is not as much flexibility for wow. For example, the champion class can spec for single target damage, aoe damage (this is it's specialty), and tanking. Since moria champs have become much more sought after as tanks than the hardcore guardian tanks because they can switch it up and deal some damage, making things go faster. Rune-keepers can spec for different types of magic damage (regular and dot mostly) or they can spec for hots (heals over time). Still, as I already stated, the flexibility is probably not what it is in wow. It doesn't really matter to me though.