Yes, AS 3 is quite a jump from AS 2 and I'm sure a lot of people have thought if it is really worth it to learn. I have actually read several different articles on the topic and at the end most people say that AS 3 is the way to go. And it will probably be around for a while so might as well learn it earlier than later
as3 is just very object oriented. Where AS2 was oop but allowed for a lot more slack, AS3 forces oop a bit more but results in much more consistent, and faster executed code. Also there are some functions that can only be achieved with AS3