Build Pack scripts such as Builder's Buddy and Rez Faux work differently from the way that the Prim Composer LSL rezzer currently works.
With a build packer, you take the already rezzed linksets, put a script in them, and then drop them in the rezzer. As a result, there is 1 script per linkset.
In the current version of the LSL rezzer, the build is represented as data in notecards. The rezzer reads the data and then rezzes an unformed linkset with the same number of prims as specified by the data. Then, each prim has to read the data that describes it and transform itself into the appropriate shape. Each prim will start out as a cube and then transform itself into a sphere or a sculptie as required by the data.
Thus, instead of having 1 script per linkset (as in build packers), there is 1 script per prim in the Prim Composer LSL rezzer.
This puts considerably more stress on the scripting engine and the server as a whole.
In addition, because build packers already have the fully formed linksets inside them, the rezzed child prims don't need to move relative to the root prim. They are already in their correct position. Thus, a build packer would not be affected by the child movement restrictions in 0.6.7.
Both techniques have advantages. There are three steps to importing a packaged build into Second Life or OpenSim: uploading, packaging, and rezzing.
The build packer technique has advantages in rezzing. It rezzes faster and puts less stress on the server during rezzing. However, uploading and packaging takes longer.
A notecard-based rezzer such as the current LSL rezzer is faster and less stressful on the server during the upload and packaging phases, but is slower and more stessful during rezzing.
I plan to implement both techniques eventually, but the notecard-based rezzer is what we have now. The reason that I started with this technique is because it can dramatically speed up uploading and packaging for builds with thousands of prims.