Many Python modules are not (yet) available to Python 3.x. So it is better to have both Python 2.6/2.7 & 3.x on Sailfish OS. More programming languages and tools = more software will be written / ported to Sailfish OS. Package dependencies hell ? Sorry, but hell is Android which doesn't have dependencies at all and the developer has to included everything inside de .apk (Python, Qt, PyQt/PySide/PyOtherSide, NumPy, MatPlotLib, etc). Something that takes 1h developing directly on Nokia N900/N9 (including packaging and sending to repository) can take many hours using full Android SDK on PC.