Foreign policy requires a lot of guesswork. Will carrots or sticks work best in a given problem area? Do threats promote conciliation or resistance?
In principle, I believe that it often helps to back diplomacy with the threat of force. But the threat has to be credible, and carrying it out shouldn't undermine the basic policy goals. That's often hard to accomplish in practice.
American lawmakers have a long history of writing conditions into foreign aid legislation. Most of those conditions are helpful in deterring foreign governments from doing things we strongly oppose -- like abusing human rights or supporting terrorists or overthrowing fairly elected democratic governments. On the other hand, Congress' efforts to prevent Pakistan from acquiring nuclear weapons, when they failed to achieve their primary purpose, forced the US government to halt arms deliveries that even today poisons US-Pakistani relations.
There's a good piece in the National Journal about some of the pending conditions on US aid and the dilemmas they entail. One analyst is quoted as saying that conditionality "is a gun with one bullet in it." What she doesn't say is that that bullet may shoot us in the foot.
I sympathize with lawmakers who want to use conditions as pressure and leverage. But I also sympathize with executive branch officials who may need wiggle room -- the right to waive the conditions -- if that can prevent self-inflicted wounds.