I mean that in TF2, the Medic always has someone to buddy up with where his existence will automatically improve their game while their existence improves his game.
In other games, though, you can't always be guaranteed to find a buddy. Imagine being a pure healer at a level where there's basically one or two other players and they have no problem soloing and so don't feel the need to split experience.
Also, dual specs are a recent trend and so far I think only WoW has them. Even "pure" healers in WoW have some inherent offensive capability, though, since damage and healing are powered by the same stat.