<blockquote>"The library approach is the simplest and works by calling functions in a library that has been accelerated by someone else. Many of the latest math libraries belong to this category. As long as most of the computation takes place inside the library code, this approach is very successful. It allows a few expert library writers to enable the acceleration of a large number of applications." -http://www.hpcwire.com/2014/01/09/future-accelerator-programming/</blockquote>
Since it is trivial to use, programmers will employ the library approach whenever possible. However, this hinges on the availability of appropriate library functions, which are easy to provide in well-defined and mature domains such as standard matrix operations (BLAS) but hard to do for emerging areas or unstructured computations.