> It doesn't necessarily seem too dangerous for one package, but it is something of a slippery slope.
This just looks like the same trade-off as branching or forking source code. Divergence is a slippery slope but sometimes you need the extra speed. Instead of a "pie in the sky" approach better acknowledge this need for speed so the acceleration is kept under control and things can eventually fall back on mainstream/upstream.
I do not think anyone likes the hassles of bundling libraries. Once a library becomes rock-solid all developers eventually rely on the version provided by the operating system. I just ran "ldd" on the latest stable firefox binary and the number of "libXXX" files bundled is way smaller than the total.