Why is BuLi a better nucleophile than base? In Carey, Sundberg Advanced Organic Chemistry it is written:


  The relative slowness of the abstraction of protons from carbon acids by organolithium
  reagents is probably also due to the compact character of the carbon-lithium
  clusters. Since the electrons associated with the carbanion are tightly associated with
  the cluster of lithium cations, some activation energy is required to break the bond
  before the carbanion can act as a base. This kinetic sluggishness of organometallic
  compounds as bases permits important reactions in which the organometallic species
  acts as a nucleophile in preference to functioning as a strong base.


Won't the same reasons apply for nucleophilic displacement? Compactness of the aggregates should affect nucleophilicity, too. So why will BuLi add to a carbonyl instead of deprotonating the alpha carbon atom?
This is a trick question, which explains why the first sentence from the book takes so many precautions. It is very difficult to provide a general explanation because there are many factors influencing the result.


Solvent: the solvent is extremely important in such reactions. It
plays an important role in the order of oligomerization of the
organolithium reagent; monomers, dimers, tetramers or polymers. Of
course, the reactivity is really different.
Co-solvent: a co-solvent like HMPT or TMEDA can drastically change
the course of the reaction by changing the order of oligomerization.
Temperature: it is well known that n-butyllithium is a very strong
base at low temperatures (&lt;-60°C) but will become a good nucleophile
at higher temperatures. It can even open a THF ring.
Thermodynamic or kinetic control: organolithium compounds are
usually used under kinetic control, but...
HSAB: how the oligomerization affects the hardness/softness of the
base and its nucleophilicity?
3D shape of the organolithium compound: it greatly influences the
shape and order of oligomers.
...


So now you feel completely depressed, I can try to answer your question. I think that a key factor is actually that in a cluster (dimer, tetramer and so on), charges are more diluted which enhances the nucleophilic behavior. Also, the geometry of the cluster and of its 3D-complex with the substrate could explain why a reaction is favored over the other.