See Symfony's (biased, but true) answer:
http://symfony.com/why-use-a-frameworkOr look at what they say at StackOverflow: http://stackoverflow.com/questions/279003/why-do-i-need-to-u...
You should replace trust with use and hopefully you'll see why a solid framework will help you focus on what you're trying to achieve instead of fixing security holes you didn't think of.