Oh, and one general negative is having a duplicate of a live tree.
That tends to get a bit problematic, and includes the need for
periodic merging from the main source. Someone will have to commit to
keeping a vendor import going.
If we duplicate the necessary parts of boost, the only time we need update is if we want to use new features or bugs are found that affect our usage. If I (and maybe Michael) are the only ones using it, this shouldn't be much of a problem. If many others start using it, then we're better off making boost an external dependancy - there're distributions for plenty of platforms, eg debian (stable), redhat, osx, windows, etc...
At one point I think there were concerns in regards to platform
coverage. Various people build Inkscape on a very wide range of
platforms and compilers, so we end up with a bit more concern than
some other projects.