If distro X has critical bug in something like Chromimium, it will prioritise the system library update because end users will notice the problem. That also generates pressure on upstream. Freezing someone elses project by forking your own copy, fixing a bug, and then hiding the problem by shipping a private version, just hides the bug.
You also carry the cost of bugs that other projects expose, which you haven't found yet (even porting to new platforms). With private copy you'll have the cost of finding, isolating & patching those to eventually, possibly having to re-integrate a whole slew of patchsets.