Li Shenron
Legend
Seems like the warlock's 'bond' is more current and open to negotiation. A cleric is just supposed to have faith. The warlock's relationship is more mercenary - 'service' for knowledge/power.
It's the kind of thing that can fade into the background if the player or DM don't feel like dealing with it - assuming it wasn't used as some sort of clumsy balancing mechanism, that is.
I agree that it should be seen as a flexible idea that can be enforced or ignored, depending on your preferences. And I also don't think there's a wanted balancing mechanism.
I might be wrong, but I used to think the original inspiration/concept of the Cleric is that of a mix between a crusader (hence the fighting skills) and a saint (divine spells = miracles), her ability fueled by her faith*, while that of the Warlock is the witch (in medieval terms, i.e. consorting with the devil, and because of that capable of sorcery).
*most commonly this is interpreted in RPG as "stick to your religion's agenda or be stripped of your magic powers", but many times I've told my players that the truth might be more mysterious, and that a Cleric might actually not know why she can heal the sick and resurrect the dead, only believe it's coming from her God while it might be just coming from herself