Note that this is mostly an issue for Sublime Text 2. Sublime Text 3, which is in beta, has a much more consistent Python 3 embed.
Unfortunately, ST3 is in one of those death-march betas that have been ongoing for years, fragmenting the plugin space. If you write a plugin for ST, you either need to write Python that simultaneously targets Python 2 and Python 3 (possible, but restrictive), or you need to maintain two versions of your plugin.
As of earlier in the year, over 80% of Package Control users were on ST3. ST2 usage seems to be waning pretty significantly. It will be nice once package developers can use all of the goodies in Python 3 and not worry about the ancient Python 2.6.
We did however have to use external python for the common python core instead of just importing the package, mainly because of the lack of ssl support in the embedded python.
Unfortunately, ST3 is in one of those death-march betas that have been ongoing for years, fragmenting the plugin space. If you write a plugin for ST, you either need to write Python that simultaneously targets Python 2 and Python 3 (possible, but restrictive), or you need to maintain two versions of your plugin.