You're building addons/features for XF and they are using a Zend Framework for their back-end. If i hired a web developer expert in using the Zend Framework, it will be very easy for him or her to code any addon or feature for XF b/c it is using Zend framework for its back-end.
I think you need to understand what the framework is. The framework is a set of common, every day routines that are used in just about every program out there. The entire idea behind the framework IS to speed development without having to write those from scratch.
Kier gave an excellent example in an interview. The DB routines in the Zend Framework. Why on earth would a developer want to reinvent the wheel and write their own DB routine when it's all available in a package?
In my case a lesser example would be in one of the first add-ons I released for XF. I could not find a routine to check if a node was a forum. So I wrote my own routine always knowing there had to be one in XF and to keep an eye out for it. A while later while working on something totally unrelated I found it and had one of those 'I could've had a V8' moments. The code I wrote was removed and replaced with the call to XF.
Compare the Zend Framework to Microsoft's .NET framework or C++ redistributables. Just about every program on your computer (if it's a windows computer) uses those. And for sure every windows program I've ever written use those. They are all common routines that interface with windows. People could write their own, but why on earth would they want to? There is nothing wrong with using them and in many cases they are required.