The middle manager developer is a position to fill on the FLOSS development stack. Is a developer that knows the project internals however the focus is more consistant with the middle ground. The ground between the rookie development and the outside development and the project internals.
Sometimes the internals represent too much of a learning curve for the more demanding hacker. So having a straight development vision that can recoup on how the development flow gets done in open source projects make the resources more efficient and optmized. When mature code is developed getting to the right documentation source makes the difference between a weekend hack and a month hack.
So for example in a big project like Apache or OpenOffice.org with million of lines of code, a hacker might be a bit lost trying to find a solution to solve a problem or to start a fundamental hack. Most of the times, the developer might find himself lost trying to identify the right development group or branch. Low level developers might be busy trying to fix issues within the core, this makes a gap between the potential new developer who isn’t verse on the code of the project yet his knowledge might help even low level developers to spur a solution.
This is where the gap might be cover by developers that understand the code and help new developers to find their right branch and hack on it with little discovery time invested.
One of the things that has point this issue is the GSOC which intends to push the development of code but also on motivating students to become FLOSS developers. In OpenOffice.org there are programs like the Education Project which push developers to get into the source. And also sandbox the development so it can be mentored and also pushed upstream on a controlled environment.