Key point of this framework are ability to swtich between canvas (HTML5 anyone?) and normal rendering and extreeeme ease of use. To demonstrate that I've done a little HelloWorld example for SpriteJS, you can download it here.
I don't know future, but if this kind of libraries can provide secure and fast deploy way for games both players and developers will have great convenience, for example:
- more platforms => more players (keep in mind new smarthphone, even IPhone or Htc),
- fast time in developing => no need to compile => more time for games,
- browsers => no more forced on one os => more players (I'm watching you linux-gamer, I know that you exist, even for casual game!)
As written before, I believe in this little "market" of open sources engines for videogame to place side by side with business options, why not?