Function composition has one *huge* benefit over chaining APIs: it allows *any* combination of JavaScript functions to be stuck together to form a pipeline. One could create a pipeline involving a function from library A, a function from library B, a function from library C, and a built-in function. With a chaining API one is restricted to the set of functions available on the object facilitating the chaining.

I found this to be a problem when using Underscore. This would be fine for a pipeline comprised solely of Underscore functions (represented here by lower-case identifiers):

```javascript
_(x)
.a(...)
.b(...)
.c(...)
.value()
```

"Foreign" functions (represented here by upper-case identifiers) break the pipeline:

```javascript
_(D(_(x)
    .a(...)
    .b(...)
    .c(...)
    .value()))
.e(...)
.f(...)
.value()
```