It may be seen as a yet another argument in favor of dynamic typing/monkey patching. I'll put this issue aside. I think that the thin interface perspective is just as interesting.
With thin interfaces an object provides only a small set of services. This leads to layering of abstractions: A new abstraction provides a new set of services on top of existing ones. We end up with a hierarchy of abstractions, each one tailored for a different type of usage.
With fat interfaces the API tends to evolve into a single uber-abstraction. As this debate shows this one-size-fits-all approach may be either good or bad, depending on your point view. Larry Page witnessed the negative side. According to his question, the underlying abstraction failed to provide the service he needed. It is just like you are given this big machine to help you do something, but it does not do exactly what you need. You can either disassemble the machine and build it all over again which is a lot of work (after all, it is a big machine) or you can build a new machine from scratch. Either way, it's a lot of work. If you already rely on this machine for other purposes then even replacing it will be expensive. As Jake Blues puts it, you find that "you're really up Shit Creek".
It seems that Shit-Creeks like this are quite rare with thin interfaces. Instead of one abstraction with many services you have many abstraction each providing a small set of services. Pluralism of abstractions promotes modularity because you can assemble them according to your needs. Tweaking is easier since you're not as bounded by contracts because each contract is relatively small.
As a direct result, thin interfaces put you, the client, in charge. This is a more democratic approach to programming. Let's spread the power around.