At least for Python the standard .pkg's available from python.org have always been very useful for me. Especially since you can install as many versions as you want simultaneously since they install in separate folders. As a tester who used to have to keep installs of lots of versions this was very helpful.
I should know the situation for Ruby as well... but don't remember anymore.
Sure but it's very rare to update python on the desktop (not on the server). Installing a new version in parallel is common, but once you get a 3.x rolling, you seldom have the need for a 3.x.y unless you encounter a bug or are very security conscious.
On the other hand, installing xcode is really a pain.
I should know the situation for Ruby as well... but don't remember anymore.