Day Late for this,
Well Spines is far superior than what ever Unity2D will give you. But like what Nate said, Its workflow and how well the tool helps you. This is going to have to be something you have to figure out. If you used spine before it won't feel any different over in unity, You can still do everything from Loading an animation, to playing one. Spine does however become more easier to use in unity because of the Heiarchy system, The Umbra Occlusion system and some fancy mono/C# serialization via scripts. Unity2D can also help compress your Spine Texture! Very nice feature.
But Unity2D uses there own animation system that is a little to not very nice at all, There node graph system would be somewhat helpful in a 3D game but in 2D, Not so. They also kind of expect you to use Sprite Sheets.... For everything. Unity2D Dev Team Ya, Lets assume the entire market is still somewhere in the limbo of the 80's and 90's.... Though I would be a little hypocritical considering I'm working on a game that has pixel art however I do use spine for making the animations(No SpriteSheet!)
If you have any specific questions(about Unity), you can PM or Email me(<removed>).