i came across this article by H.D. Moore about how assembly is key in penetration tests. I think, if you want to perform an in depth pentest, you have to have a solid knowledge of such programming language to, if necessary, write your own exploits/code to perform that extra step to succesfully complete your assessment. even with todays tooling and automated script you need to know how computers work inside out.

the link:
http://blog.metasploit.com/2010/04/penetration-testing-learn-assembly.html