I thought about that, and it's something if I had to do it over again I might very well change, but at the same time I don't necessarily want to get into the habit of role-playing for the characters.
Another thing I'd consider taking into account is alignment.
I think (as a very, very broad generalisation) a lawful (particularly lawful evil) creature will be concerned with the letter of a command/compulsion/contract, while a chaotic creature will tend to identify more with the spirit.
I think if you lay a
Geas or
Planar Binding on a demon, it'll be upset, and if it manages to
break the spell, you'll be in big trouble, but it'll generally do what you intend.
If you do the same thing to a devil, I should imagine they'd make an outward show of cooperating... but in their heads, they'll be racing through every possible interpretation of every single word, looking for a way to fulfil your command literally, while at the same time completely screwing with the intent.
Obviously, the more intelligent the creature is, the better chance it has of
finding the loopholes.
By the same token, if you're allowing a player to state the intent of the command, and then wording it for him on the basis of the character's intelligence, I'd be inclined to let a lawful character generate a more airtight phrasing than a chaotic.
-Hyp.