one problem with canvas is there are use cases where you are going to want to export as a vector graphics file. This is doable (but somewhat non-trivial) in an SVG element, but basically impossible for a canvas element.
actually, for example, EaselJS is written up very neatly and IS easy to work with, so I see absolutely no problem writing a custom adapter which would recursively reiterate the EaselJS Stage object (toString() method) and parse out all the shapes and what-not to whatever format you could possibly want.
sure, but with svg you already have that, written by the good folks at mozilla/google/apple.
I don't know how easelJS works, but the other really nice thing about svg is that modification of some attributes (color, stroke-width, etc.) is done using CSS, so if you, say, wanted to have the user be able to customize the interface in general cases (I was using this to annotate DNA which usually are done with arrows - one user might want genes in yellow and promoters in green, another vice versa, etc) that's a trivial implementation; no need to write a custom lookup table.
