right now, all animation interpolation is linear

linear interpolation is crap for animating characters, it completely ignores the laws of physics, as each movement does not take acceleration, deceleration and momentum into consideration
many animation designers swear not to use cocostudio, as it does not support better interpolations

Cocos2d already have a complete Ease functions available, it is pretty easy to copy it, therefor it is not a big task

provide Ease methods as a choice for animation designers, which affects the end result.
This is a feature of a prime importance, that once done, can help people creating much better quality animations, and thus much more important than optional features that people might/might-not use such as collision detection, and Flash vector etc