I know why this is good in general: faster security fixes, easier packaging, more features. However, I'm trying to persuade some co-workers that we don't need to bundle a library with our program. It will not work without this library, but the library has been stable for a while now and will remain so for the foreseeable future. I don't see any reason NOT to unbundle it.
What arguments could I use to persuade them?
My specific situation is this: I'm working on SymPy, which is an open-source Python library for symbolic mathematics. A core part of it is mpmath, which is a library for multi-prevision floating point arithmetic. SymPy doesn't work without mpmath, there is no alternative. As such, it has been bundled with SymPy since the start (I was told that there were usually small incompatibilities to fix every time a new version is imported). It should also be noted that the developer of mpmath used to be involved in SymPy development. There is now an issue on unbundling mpmath, you can read it all here.
To summarize the discussion there:
Somewhat easier porting to Python 3 (minor argument IMHO)
Easier packaging for distributions
Faster (security) feature updates to users
"Packaging and handling dependencies are hard problems, but they are solved. This is definitely not an area where we should do our own thing."
Installation. It's easy on Linux, harder on Mac and very hard on Windows. Lack of su access and other problems.
it is an integral part of SymPy, i.e. sympy does not work without it (at all)
there is no other package, that can do the job of mpmath
"When I, as a user, download sympy, I expect it to just work."
That's my specific situation, but I'd accept an answer that provides a good, general answer as well.