Usually when PHP developers embark onto a medium scale project they find a framework that fits their needs. Frameworks allow developers to use a highly tested and feature packed set of tools that only speed up development.
However, once companies or developers evolve to a larger company this little framework has a few problems they might encounter.
- Its not your product. You could spend time fixing their bugs, but then when a new version comes out. You have some conflicting files. You could never update, but then you loose out on stability and security fixes. As well as maybe some feature enhancements.
- Who's to say this framework will always be available? If you are using an open sourced product, it might be dropped by the developers and the community would be forced to hold it. Maybe that's something you don't want to factor into your business plan.
- Maybe you are switching your server fleet over to litespeed or something else that might have conflicts with this framework. You are then limited by your framework.
- The only way you become an improved programmer is by programming a lot. When using a framework you just pass x and y to a and get back d. You have some knowledge of what goes on, but the true knowledge would be obtained if you programmed that function.
- There are times to re-invent the wheels and other times to use what works.