Tomas, if your framework is properly designed, you don’t need to rebuild everything each time you want to add a new feature.
I have used somebody else’s frameworks several times and most of the times there is so much automagic going on that they go against you. You need to learn how to do things their way, look for strange and unpredictable/undocumented behaviour and etc, whereas if you know what you’re doing (here’s the basic premise) you just do it and jump to the next item in the to do list.
The security aspect is quite important as well. With code that you’ve developed, you know exactly what is going on. With somebody else’s code, you either audit everything or have hope ;)