IMHO, the only valid reason for "roll your own" is if the application needs 50x+ performance than what would be possible using existing solutions (as is the case described in this blog post). Otherwise follow the rules:

The three rules of software development:

1. Don't write code as there is a library that already does exactly what you want to do

2. Don't write code because you can combine several libraries to do what you want to do

3. Don't write code because there is some mega-framework, one subset of which does what you want to do