Parser generators are useful to explore the problem space. It allows for a higher-level thinking. I always ended up rewriting the parser by hand but I'm not sure I could achieve the same result without the first prototype.
This is the way I usually end up doing things as well. I originally tested out the text grammar for later.js using a parser generator. Then once I worked out the kinks and played around with it a bit, I rewrote the parser so that it was specific to the grammar I needed with some additional flexibility that wasn't available from the generator.